Multi-layer piezoelectric element

ABSTRACT

To provide a multi-layer piezoelectric device having excellent durability in which the amount of displacement does not change even when the piezoelectric actuator is subjected to continuous operation over a long period of time under a high voltage and a high pressure, the multi-layer piezoelectric device comprises a stack formed by stacking piezoelectric layers and internal electrodes alternately one on another and external electrodes formed on a first side face and on a second side face of the stack, wherein one of the adjacent internal electrodes is connected to the external electrode formed on the first side face and the other internal electrode is connected to the external electrode formed on the second side face, while content of alkali metal in a range from 5 ppm to 300 ppm is contained.

TECHNICAL FIELD

The present invention relates to a multi-layer piezoelectric device andinjection apparatus, for example, fuel injection apparatus of automobileengine, liquid injection apparatus of ink jet printer or the like or adrive unit used in precision positioning device or vibration preventingdevice for an optical apparatus, and to a multi-layer piezoelectricdevice used as a sensor element mounted in combustion pressure sensor,knocking sensor, acceleration sensor, load sensor, ultrasound sensor,pressure sensor, yaw rate sensor or the like, or used as a circuitcomponent mounted in piezoelectric gyro, piezoelectric switch,piezoelectric transducer, piezoelectric breaker or the like.

BACKGROUND ART

Multi-layer piezoelectric actuators constituted from piezoelectriclayers and internal electrodes stacked alternately one on another havebeen known as an example of the multi-layer piezoelectric device. Themulti-layer piezoelectric actuators can be divided into two categories:fired-at-once type and stacked type which has such a constitution aspiezoelectric porcelain and internal electrode sheets are stacked one onanother alternately. When the requirements to reduce the operatingvoltage and the manufacturing cost are taken into consideration, themulti-layer piezoelectric actuator of fired-at-once type is moreadvantageous for the reason of smaller layer thickness and higherdurability.

FIG. 2 shows a multi-layer piezoelectric device of the prior artdisclosed in Patent Document 1, which is constituted from a stack 200,that is formed by stacking piezoelectric layers 21 and internalelectrodes 22 alternately one on another, and external electrodes 23formed on a pair of side faces that oppose each other. While the stack200 is formed by stacking the piezoelectric layers 21 and the internalelectrodes 22 alternately one on another, the internal electrodes 22 arenot formed over the entire principal surfaces of the piezoelectriclayers 21, but have a so-called partial electrode structure. Thepiezoelectric layers are stacked in the so-called partial electrodestructure such that the internal electrode 22 is placed in every otherlayer in a staggered manner so as to be exposed alternately at the leftthen at the right on different side faces of the stack 200. Inactivelayers 24, 24 are stacked on both principal surfaces of the stack 200 inthe direction of stacking. Then the external electrodes 23 are formed sothat the internal electrodes 22 that are exposed on the pair of opposingside faces of the stack 200 are connected to each other, therebyconnecting the internal electrodes 22 in every other layer.

In case the multi-layer piezoelectric device of the prior art is used asa piezoelectric actuator, lead wires are soldered onto the externalelectrodes 23 and operated by applying a predetermined voltage betweenthe external electrodes 23. In recent years, since it is required tomake a compact multi-layer piezoelectric device capable of achieving alarge amount of displacement under a high pressure, it is in practice tocarry out continuous operation over a long period of time with a higherelectric field applied.

The multi-layer piezoelectric device is manufactured as follows. First,an internal electrode paste is printed in the pattern of a predeterminedelectrode structure as shown in FIG. 2 on a ceramic green sheet thatcontains the material of the piezoelectric layer 21, stacking aplurality of the green sheets coated with the internal electrode pasteso as to form a multi-layer compact and firing the compact thereby tomake the stack 200. Then the external electrodes 23 are formed on a pairof side faces of the stack 200 by firing, thereby to make themulti-layer piezoelectric device.

During manufacture of the stack 200 of the prior art, alkali metal maymix into the material. That is, the stock material mixed into the greensheet of the piezoelectric layer 21 and the binder may contain an alkalimetal in the form of oxide, carbonate or nitrate, or may mix as aninevitable impurity. While a glass powder may be added to the stockmaterial of the piezoelectric layer 21 for the purpose of improving theease of sintering thereof, the glass powder often contains oxides ofalkali metals. Moreover, the alkali metal may also mix into the materialfrom crushing balls used in crushing the stock material to make thepiezoelectric layer 21 or from the firing atmosphere through diffusion.

Halogen elements may also mix into the material. That is, the stockmaterial used to make the piezoelectric layer 21 and the binder mayinclude halogen elements in the form of fluoride, chloride, bromide,iodide or astatine compound, or mixing therein as an inevitableimpurity. Also in the manufacturing process, use of water duringcrushing or storage of the stock material used to make the piezoelectriclayer 21 over a long period of time may lead to mixing of halogenelements into the material. Moreover, halogen elements may also mix intothe material from crushing balls used in crushing the stock material ofthe piezoelectric layer 21 or from the firing atmosphere throughdiffusion.

The alkali metals and halogen elements may also mix into the material inthe form of compounds such as NaCl from human bodies.

The internal electrode 22 has been formed from an alloy of silver andpalladium and, in order to fire the piezoelectric layers 21 and theinternal electrodes 22 at the same time, composition of metals containedin the internal electrode 22 has been set to 70% by weight of silver and30% by weight of palladium (refer to, for example, Patent Document 2).

The internal electrode is made of metal compound that containssilver-palladium alloy instead of pure silver because, when a voltage isapplied between the pair of opposing electrodes that are made of silverwithout palladium content, the so-called silver migration occurs inwhich silver atoms migrate from the positive electrode to the negativeelectrode of the pair of the electrodes along the device surface. Silvermigration occurs conspicuously particularly in an atmosphere of hightemperature and high humidity.

During manufacture of the internal electrodes 22, alkali metal may mixinto the internal electrodes 22. That is, the stock material of theinternal electrodes 22 and the binder may include alkali metal in theform of oxide, carbonate or nitrate, or mix therein as an inevitableimpurity. While a glass powder may be added to the stock material of theinternal electrodes 22 for the purpose of improving the ease ofsintering, the glass powder often contains oxides of alkali metals.Moreover, alkali metal may also mix into the material from crushingballs used in crushing the stock material of the internal electrodes 22or from the firing atmosphere through diffusion.

Halogen elements may also mix into the internal electrodes 22. That is,the stock material used to make the internal electrodes 22 and thebinder may include halogen elements in the form of fluoride, chloride,bromide, iodide or astatine compound, or mix into the material as aninevitable impurity. Also in the manufacturing process, storage of thestock material used of the internal electrodes 22 over a long period oftime may lead to mixing of halogen elements into the material. Moreover,halogen elements may also mix into the material from the firingatmosphere through diffusion.

Both the alkali metals and halogen elements may also mix into thematerial in the form of compounds such as NaCl from human bodies.

During manufacture of the external electrodes 23 of the prior art,alkali metal may mix into the external electrodes 23. That is, the stockmaterial of the external electrodes 23 and the binder may include alkalimetal in the form of oxide, carbonate or nitrate, or mix therein as aninevitable impurity. While a glass powder may be added to the stockmaterial of the external electrodes 23 for the purpose of improving theease of sintering, the glass powder often contains oxides of alkalimetals. Moreover, alkali metal may also mix into the material fromcrushing balls used in crushing the stock material of the externalelectrodes 23 or from the firing atmosphere through diffusion.

Halogen elements may also mix into the external electrodes 23. That is,the stock material used to make the external electrodes 23 and thebinder may include halogen elements in the form of fluoride, chloride,bromide, iodide or astatine compound, or mix into the material as aninevitable impurity. Also in the manufacturing process, use of water inthe mixing and crushing process or storage of the stock material used ofthe external electrodes 23 over a long period of time may lead to mixingof halogen elements into the material. Moreover, halogen elements mayalso mix into the material from crushing balls used in crushing thestock material of the external electrodes 23 or from the firingatmosphere through diffusion.

Both the alkali metals and halogen elements may also mix into thematerial in the form of compounds such as NaCl from human bodies.

When the multi-layer piezoelectric device is manufactured by firing thestack constituted from a plurality of the green sheets formed from thestock material of the piezoelectric layer 21 and the binder whereon thepaste constituted from the stock material of the internal electrodes 22and the binder is printed thereon, the alkali metal contained in thepiezoelectric layer 21 and in the internal electrodes 22 may diffusefrom a portion of higher concentration of alkali metal into a portion oflower concentration. Distance over which the diffusion propagates variesdepending on the firing temperature, duration of firing and the ratio ofconcentrations. The halogen element contained in the piezoelectric layer21 and in the internal electrodes 22 may also diffuse from a portion ofhigher concentration of halogen element into a portion of lowerconcentration. Distance over which the diffusion propagates variesdepending on the firing temperature, duration of firing and the ratio ofconcentrations.

Also when the paste constituted from the stock material of the externalelectrodes 23 and the binder is printed and fired on the pair of sidefaces of the stack 200, alkali metals contained in the externalelectrodes 23 and in the piezoelectric layer 21 that is in contact withthe external electrodes 23 may diffuse from a portion of higherconcentration of alkali metal into a portion of lower concentration.Distance over which the diffusion propagates varies depending on thefiring temperature, duration of firing and the ratio of concentrations.In addition, alkali metals contained in the external electrodes 23 andin the internal electrodes 22 that is in contact with the externalelectrodes 23 may diffuse from a portion of higher concentration ofalkali metal into a portion of lower concentration. Distance over whichthe diffusion propagates varies depending on the firing temperature,duration of firing and the ratio of concentrations.

Similarly, halogen element contained in the external electrodes 23 andin the piezoelectric layer 21 that is in contact with the externalelectrodes 23 may also diffuse from a portion of higher concentration ofthe halogen element into a portion of lower concentration. Distance overwhich the diffusion propagates varies depending on the firingtemperature, duration of firing and the ratio of concentrations.Similarly, halogen element contained in the external electrodes 23 andin the internal electrode 22 that is in contact with the externalelectrodes 23 may also diffuse from a portion of higher concentration ofthe halogen element into a portion of lower concentration. Distance overwhich the diffusion propagates varies depending on the firingtemperature, duration of firing and the ratio of concentrations.

Alkali metals are very effective in assisting the sintering reaction ofceramics materials, and have long been used as the sintering assistingagent. However, excessive amount of alkali metal results in dielectricloss of high frequency energy. Therefore, it has been a common practiceto decrease the content of alkali metal in order to decrease thedielectric loss in ceramic materials used in IC packages which sufferincreasing transmission loss of signals when the dielectric loss of highfrequency energy increases, and capacitors which suffer decreasing Qvalue and increasing heat generation when the dielectric loss of highfrequency energy increases. In the multi-layer piezoelectric device, incontrast, the device is driven with a high DC voltage and operates at alow frequency of 1 kHz or less, unlike the applications described above.Therefore, high frequency dielectric characteristic is not of highpriority for the multi-layer piezoelectric device. Since it has beenrequired to form the piezoelectric layer 21 from a dense sinteredmaterial in order to achieve high insulation with regards to highvoltage, alkali metal has been used as the sintering assisting agent.

Composite perovskite type compound containing PbTiO₃—PbZrO₃ (hereinafterabbreviated as PZT) as the main component has been used as ceramic orpiezoelectric ceramic material. Most of the components of thesematerials are ceramic materials, which are formed by forming the stockmaterial or calcined powder into a compact of predetermined shape andfiring the compact at a high temperature. These piezoelectric ceramicmaterials have been made so as to provide various properties for suchapplications as actuator, ceramic filter and piezoelectric buzzer, byadjusting the proportions of the components. For example, apiezoelectric actuator consumes less power and generates less heat thanthe conventional electromagnetic actuator of the prior art made from amagnetic material with a coil wound around thereof, and has excellentproperties such as fast response, larger amount of displacement, smallersize and smaller weight. However, PZT ceramics has drawbacks such as low4-point bending strength that is about 100 MPa and susceptibility tocracks and breakage during machining.

Patent Document 3 discloses PZT piezoelectric ceramics that contains0.01 to 0.3% by weight of Fe, 0.01 to 0.04% by weight of Al and 0.01 to0.04% by weight of Si as auxiliary components for the purpose ofsuppressing cracks and breakage from occurring during machining with agrinder.

With the PZT piezoelectric ceramics disclosed in Patent Document 1, Aland Si added as the auxiliary components tend to form liquid phase inthe sintering process, resulting in a glass phase that containsPbO—Al₂O₃—SiO₂ in the grain boundary after the sintering process.Accordingly, a dense sintered material can be made with crystal grainsgrown therein at a temperature lower than that for sintering apiezoelectric ceramic material that does not include such auxiliarycomponents. As a result, crack and breakage can be suppressed fromoccurring during machining with a grinder and the glass phase has higherrupture toughness than the perovskite type compound, thus increasing therupture toughness of the sintered material.

Multi-layer piezoelectric actuators constituted from piezoelectriclayers and internal electrodes stacked alternately one on another havebeen known as an example of the multi-layer piezoelectric device. Themulti-layer piezoelectric actuators can be divided into two categories:fired-at-once type and stacked type in which piezoelectric porcelain andinternal electrode sheets are stacked one on another alternately. Whenthe requirements to reduce the operating voltage and the manufacturingcost are taken into consideration, the multi-layer piezoelectricactuator of fired-at-once type is more advantageous for the reason ofsmaller layer thickness and higher durability.

-   Patent Document 1: Japanese Unexamined Patent Publication (Kokai)    No. 61-133715-   Patent Document 2: Japanese Unexamined Utility Model Publication    (Kokai) No. 1-130568-   Patent Document 3: Japanese Unexamined Patent Publication (Kokai)    No. 14-220281

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, the multi-layer piezoelectric device used in an environment ofhigh temperature and high pressure has such a problem that, whentemperature of the multi-layer piezoelectric device rises, twoimpurities contained in the multi-layer piezoelectric device are ionizedthereby causing a change in the specific resistance of the multi-layerpiezoelectric device that in turn changes the amount of displacement.

Accordingly, when the conventional multi-layer piezoelectric device isused in such an application as fuel injection apparatus for automobileengine over a long period of time, there is a problem that the amount ofdisplacement gradually changes and causes malfunction of the apparatusto occur. Therefore, it has been called for to suppress the amount ofdisplacement from changing and improve durability during continuousoperation over a long period of time.

There has also been such a problem that the amount of displacement of apiezoelectric material varies as the temperature changes, and thereforevolumetric expansion of the internal electrode occurs when the devicetemperature rises, thus causing the amount of displacement of apiezoelectric actuator to change. A change in the amount of displacementduring operation of the actuator, in turn, causes fluctuation in theload on the power source that supplies the voltage, thus placing aburden on the power source. When the amount of displacement undergoes arapid change, not only the amount of displacement deteriorates rapidlybut also the heat generated by the device exceeds the heat that can beremoved by dissipation and thermal excursion occurs, thus resulting inbreakage and failure.

In recent years, such a practice has been becoming common that apiezoelectric actuator is installed in an injection apparatus and isoperated continuously over an extended period of time with an electricfield of higher intensity being applied thereto, in order to obtain alarger displacement under a high pressure. However, when a high electricfield is applied to the piezoelectric actuator, the junction between theinternal electrode and the external electrode undergoes significantlocalized heating due to pinching of the path of electrical conduction.This results in a decrease in the capability of the piezoelectricactuator to expand and contract, thus making it difficult to operate theinjection apparatus based on the piezoelectric actuator continuouslyover a long period of time.

There has been a demand for an internal electrode that has lowerspecific resistance, in order to keep the device temperature fromrising. However, the silver-palladium alloy has specific resistancehigher than that of pure silver or pure palladium, and thesilver-palladium alloy having composition of 70% by weight of silver and30% by weight of palladium shows specific resistance 1.5 times that ofpure palladium. In addition, resistance of an internal electrode made bysintering with a lower density becomes even higher.

The piezoelectric ceramics disclosed in Patent Document 3 has a problemof chronic change in the volumetric specific resistance, which resultsin low reliability and low durability. Reliability and durability areimportant factors for the piezoelectric ceramics used in suchapplications as piezoelectric actuator installed in a vehicle.

There are also such problems as fall-off of grains that results in poorsurface roughness during machining or ultrasonic cleaning of thepiezoelectric ceramics, and damages affecting not only the surface butalso the crystal grains located inside, thus compromising thedurability.

The present invention has been made to solve the problems describedabove, and has an object of providing a multi-layer piezoelectric devicehaving excellent durability in which the amount of displacement does notchange even when the piezoelectric actuator is subjected to continuousoperation over a long period of time under a high voltage and a highpressure.

Another object of the present invention is to provide a piezoelectricceramic material having excellent durability that does not experienceinsulation breakdown even when subjected to continuous operation over along period of time under a high voltage and a high pressure, and amulti-layer piezoelectric device using the same.

Means For Solving The Problems

The inventors of the present application conducted a research to make amulti-layer piezoelectric device that can be used without undergoing achange in the amount of displacement with excellent durability, andobtained the following findings.

First finding is that the amount of displacement changes when alkalimetal is contained as an impurity in a concentration beyond a certainlevel in the piezoelectric layer 21 that has a composition where alkalimetal is not a main component. That is, more alkali metal atoms exist inthe form of ion in the piezoelectric layer that has a composition wherealkali metal is not a main component. When a voltage is applied to theexternal electrodes of multi-layer piezoelectric device made of such amaterial, and especially the actuator is operated with a high DCvoltage, the alkali metal ions migrate between the internal electrodes.Continuous operation over a long period of time under this condition isconsidered to cause the specific resistance of the multi-layerpiezoelectric device to change, thus resulting in decreasing amount ofdisplacement.

When the alkali metal ions are concentrated locally, in particular,localized short-circuiting between the internal electrodes may occur,thus interrupting the operation. Possibility of the short-circuitingbecomes higher when operated in an environment of high temperature andhigh humidity.

The problem of alkali metal is not limited to the content thereof in thepiezoelectric layer. In case alkali metal is contained in the internalelectrode, when a voltage is applied to the external electrodes of themulti-layer piezoelectric device, and especially the actuator isoperated with a high DC voltage, the alkali metal ions migrate from theinternal electrode that serves as a positive electrode through thepiezoelectric layer to the internal electrode that serves as a negativeelectrode. This phenomenon is considered to cause the specificresistance of the multi-layer piezoelectric device to change, thusresulting in decreasing amount of displacement. Moreover, in case alkalimetal is contained in the external electrode, when a voltage is appliedto the external electrodes of the multi-layer piezoelectric device, andespecially the actuator is operated with a high DC voltage, the alkalimetal ions migrate from the external electrode that serves as a positiveelectrode through the internal electrode that serves as a positiveelectrode or the piezoelectric layer to the internal electrode thatserves as a negative electrode, thereby to cause the specific resistanceof the multi-layer piezoelectric device to change, thus resulting indecreasing amount of displacement.

Thus there has been the problem of the migration of the alkali metalions in various positions, thereby to cause the specific resistance ofthe multi-layer piezoelectric device to change, thus resulting indecreasing amount of displacement. While the migration of the alkalimetal ions occurs through diffusion of the alkali metal ions from aposition of higher concentration of alkali metal ions to a position oflower concentration, a voltage applied from the outside causes selectivemigration to the negative electrode that has opposite polarity to thealkali metal ion.

Second finding is that, in case the piezoelectric layer contains halogenelement as an impurity, the change in the amount of displacement occurswhen the halogen atoms exist in the form of ion in the piezoelectriclayer. That is, when a voltage is applied to the external electrodes ofthe multi-layer piezoelectric device, and especially the actuator isoperated with a high DC voltage, the halogen elements are ionized andmetal ions migrate as the electrolyte component, thus causing thespecific resistance of the multi-layer piezoelectric device to change,and resulting in decreasing amount of displacement. When this phenomenonbecomes conspicuous, migration of metals contained in the internalelectrode and in the external electrode is accelerated and localizedshort-circuiting between the internal electrodes may occur, thusinterrupting the operation. Possibility of the short-circuiting becomeshigher when operated in an environment of high temperature and highhumidity.

The problem described above is not limited to a case where halogenelement is contained in the piezoelectric layer. When ionized halogenelements such as chlorine combine with moisture contained in theatmosphere and generate electrolyte component in the internal electrodeand in the external electrode, it has an effect similar to that ofhydrochloric acid to corrode the electrodes and cause spark when a highvoltage is applied to the device. The metal that constitutes theelectrode may also dissolve in the form of ion into the electrolytecomponent so as to form a precipitate from the metal that constitutesthe electrode and the halogen element that may cause insulation failureand eventually shutting down the operation. Furthermore, in case halogenelement is contained in the internal electrode, when a voltage isapplied to the external electrodes of multi-layer piezoelectric device,and especially the device is operated with a high DC voltage, thehalogen ions migrate from the internal electrode that serves as anegative electrode through the piezoelectric layer to the internalelectrode that serves as a positive electrode. This phenomenon isconsidered to cause the specific resistance of the multi-layerpiezoelectric device to change, thus resulting in decreasing amount ofdisplacement. Moreover, in case halogen element is contained in theexternal electrode, when a voltage is applied to the external electrodesof multi-layer piezoelectric device, and especially the actuator isoperated with a high DC voltage, the halogen ions migrate from theexternal electrode that serves as a negative electrode through theinternal electrode that serves as a negative electrode or thepiezoelectric layer to the internal electrode that serves as a positiveelectrode, thereby to cause the specific resistance of the multi-layerpiezoelectric device to change, thus resulting in decreasing amount ofdisplacement.

Thus there has been the problem of the migration of the halogen ions invarious positions, thereby to cause the specific resistance of themulti-layer piezoelectric device to change, thus resulting in decreasingamount of displacement, or when ionized halogen elements such aschlorine combine with moisture contained in the atmosphere and generateelectrolyte component, it may cause spark when a high voltage is appliedto the device, or the metal that constitutes the electrode may alsodissolve in the form of ion into the electrolyte component so as to forma precipitate from the metal that constitutes the electrode and thehalogen element that may cause insulation failure and eventuallyshutting down the operation. While the migration of the halogen ionsoccurs through diffusion of the halogen ions from a portion of higherconcentration of halogen ions to a portion of lower concentration, avoltage applied from the outside causes selective migration to thepositive electrode that has opposite polarity to the halogen ion.

When the problem of containing alkali metal and the problem ofcontaining halogen element take place simultaneously, the effects ofboth problems appear and, at the same time, when moisture deposits onthe surface of the multi-layer piezoelectric device, ionized alkalimetal forms electrolyte component that may cause spark under a highvoltage applied to the device and a salt to be formed when theelectrolyte component is dried. As a result, the internal electrode 22and the external electrode 23 are corroded and cause insulation failureand eventually shutting down the operation.

First multi-layer piezoelectric device of the present inventioncomprises a stack formed by stacking piezoelectric layers and internalelectrodes alternately one on another and external electrodes formed ona first side face and on a second side face of the stack, wherein one ofthe adjacent internal electrodes is connected to the external electrodeformed on the first side face and the other internal electrode isconnected to the external electrode formed on the second side face,while content of alkali metal in a range from 5 ppm to 300 ppm iscontained.

In the first multi-layer piezoelectric device of the present invention,the piezoelectric layer may include alkali metal in a concentration from5 ppm to 500 ppm and the internal electrode may include alkali metal ina concentration from 5 ppm to 500 ppm. Also in the first multi-layerpiezoelectric device of the present invention, the external electrodemay include alkali metal in a concentration from 5 ppm to 500 ppm.

In the first multi-layer piezoelectric device of the present invention,the alkali metal may be at least one kind of Na and K.

The first multi-layer piezoelectric device of the present invention mayfurther include halogen element in a concentration from 5 ppm to 1000ppm.

When the concentration of alkali metal contained as an impurity in thefirst multi-layer piezoelectric device of the present invention islimited within the range described above, presence of alkali metal ionsin the piezoelectric layer, in the internal electrode and in theexternal electrode is restricted. This makes it possible to maintain thetemperature of the multi-layer piezoelectric device constant even whenthe piezoelectric actuator is subjected to continuous operation over along period of time under a high voltage and a high pressure, thuspreventing the amount of displacement from changing. As a result, themulti-layer piezoelectric device having excellent durability and highreliability that can suppress malfunction of the device free andshort-circuiting from occurring, and an injection apparatus using thesame can be provided.

Second multi-layer piezoelectric device of the present inventioncomprises a stack formed by stacking piezoelectric layers and internalelectrodes alternately one on another and external electrodes formed ona first side face and on a second side face of the stack, wherein one ofthe adjacent internal electrodes is connected to the external electrodeformed on the first side face and the other internal electrode isconnected to the external electrode formed on the second side face,while content of halogen element in a range from 5 ppm to 1000 ppm iscontained.

In the second multi-layer piezoelectric device of the present invention,the piezoelectric layer may include halogen in a concentration from 5ppm to 1500 ppm and the internal electrode may include halogen elementin a concentration from 5 ppm to 1500 ppm.

Also in the second multi-layer piezoelectric device of the presentinvention, the external electrode may include halogen element in aconcentration from 5 ppm to 1500 ppm. Also in the second multi-layerpiezoelectric device of the present invention, the halogen element maybe at least one kind of Cl and Br.

When the concentration of halogen element contained as an impurity inthe second multi-layer piezoelectric device of the present invention islimited within the range described above, ionization of halogen elementin the piezoelectric layer, in the internal electrode and in theexternal electrode is restricted. This makes it possible to maintain thetemperature of the multi-layer piezoelectric device constant even whenthe piezoelectric actuator is subjected to continuous operation over along period of time under a high voltage and a high pressure, thuspreventing the amount of displacement from changing. As a result, themulti-layer piezoelectric device having excellent durability and highreliability that can suppress malfunction of the device andshort-circuiting from occurring, and an injection apparatus using thesame can be provided.

The same effect as described above can be achieved also in case thealkali metal and halogen element are contained together.

Third multi-layer piezoelectric device of the present inventioncomprises a stack formed by stacking piezoelectric layers and internalelectrodes alternately one on another and external electrodes formed ona first side face and on a second side face of the stack, wherein one ofthe adjacent internal electrodes is connected to the external electrodeformed on the first side face and the other internal electrode isconnected to the external electrode formed on the second side face,while the ratio of change in the device dimension after undergoing 1×10⁹cycles of continuous operation to the initial device dimension is notlarger than 1%.

In the third multi-layer piezoelectric device of the present invention,the ratio of change in thickness of the internal electrode afterundergoing 1×10⁹ cycles of continuous operation to the initial thicknessof the internal electrode is not larger than 5%.

The third multi-layer piezoelectric device of the present invention doesnot experience substantial change in the amount of displacement aftercontinuous operation, and therefore has excellent durability and is freefrom malfunction of the device and thermal excursion.

In the third multi-layer piezoelectric device of the present invention,when the ratio of change in thickness of the internal electrode aftercontinuous operation is set within 5%, the ratio of change in the devicedimension can be kept within 1%, thus achieving similar effect.

In the first through third multi-layer piezoelectric devices of thepresent invention, it is preferable to add an inorganic component alongwith the metallic component in the internal electrode. This increasesthe bonding strength between the internal electrode and thepiezoelectric layer, thereby preventing the internal electrode and thepiezoelectric layer from coming off each other. It is preferable thatthe inorganic component contains perovskite type oxide consisting ofPbZrO₃—PbTiO₃ as the main component.

In the first through third multi-layer piezoelectric devices of thepresent invention, it is preferable the piezoelectric layer containsperovskite type oxide as the main component.

When the piezoelectric layer contains perovskite type oxide consistingof PbZrO₃—PbTiO₃ as the main component, the piezoelectric layer and theinternal electrode can be fired at the same time, and therefore thefiring process can be reduced in time and specific resistance of theinternal electrode can be decreased.

Firing temperature of the stack is preferably in a range from 900 to1000° C.

When the deviation in the composition of the internal electrode that iscaused by the firing operation is kept within 5%, the internal electrodethat can deform in conformity with the expansion and contraction duringthe operation of the multi-layer piezoelectric device can be formed,thus making it possible to suppress the internal electrode from comingoff.

Fourth multi-layer piezoelectric device of the present invention is madeby stacking piezoelectric layers and internal electrodes alternately oneon another, wherein the piezoelectric layer contains PbTiO₃—PbTZrO₃ asthe main component and contains Si in a concentration of 5 ppm or higherand less than 100 ppm.

With the piezoelectric layer that contains PbTiO₃—PbTZrO₃ as the maincomponent and contains Si in a concentration of 5 ppm or higher and lessthan 100 ppm, glass phase is not formed in the grain boundary andchronic change in the volumetric specific resistance can be kept small.In the multi-layer piezoelectric device where the piezoelectric layersdescribed above are used, the external electrodes and the internalelectrodes do not break even when the piezoelectric actuator issubjected to continuous operation over a long period of time under ahigh voltage and a high pressure. Thus the multi-layer piezoelectricdevice having excellent durability can be provided.

In the fourth multi-layer piezoelectric device of the present invention,it is preferable that Si is segregated in the crystal grain boundary,and thickness of the grain boundary is not larger than 1 nm.

In the first through fourth multi-layer piezoelectric devices of thepresent invention, when the metal compound in the internal electrodecontains group VIII metal and/or group Ib metal as the main components,the internal electrode can be formed from the metal compound that hashigh heat resistance and therefore can be fired together with thepiezoelectric layer that has a higher firing temperature at the sametime.

In the first through fourth multi-layer piezoelectric devices of thepresent invention, proportion M1 (% by weight) of the group VIII metaland proportion M2 (% by weight) of the group Ib metal in the internalelectrode satisfy the relations 0<M1≦15, 85≦M2<100 and M1+M2=100, sothat specific resistance of the internal electrode can be kept low. As aresult, generation of heat from the internal electrode can be suppressedeven when the multi-layer piezoelectric device is operated continuouslyover a long period of time. Moreover, since the temperature of themulti-layer piezoelectric device can be suppressed from increasing, theamount of displacement of the device can be stabilized.

In the first through fourth multi-layer piezoelectric devices of thepresent invention, the group VIII metal is at least one kind selectedfrom among Ni, Pt, Pd, Rh, Ir, Ru and Os, and the group Ib metal is atleast one kind selected from among Cu, Ag and Au, and any of alloy andmixed powder materials may be used to form the internal electrode.

Further, when the group VIII metal is at least one kind selected fromamong Pt and Pd, and the group Ib metal is at least one kind selectedfrom among Ag and Au, the internal electrode having high heat resistanceand high oxidation resistance can be formed.

Further, the group Ib metal may be Cu and the group VIII metal may beNi. When the group VIII metal is Ni and the group Ib metal is Cu, stressgenerated by the displacement during operation can be mitigated and theinternal electrode that is excellent in heat resistance and in heatconductivity can be formed.

Moreover, in the multi-layer piezoelectric device of the presentinvention, it is preferable that the internal electrode includes voidsand the voids occupy 5 to 70% of cross sectional area of the internalelectrode. This decreases the restriction exercised by the internalelectrode on the deformation of the piezoelectric layer under the effectof electric field, thereby increasing the amount of displacement. Italso provides such an advantage that stress generated in the internalelectrode is mitigated by the voids, thereby improving the durability ofthe device. In addition, while heat transfer within the device ispredominantly carried by the internal electrode, existence of the voidsin the internal electrode mitigates the change in temperature within thedevice caused by rapid changes in the temperature outside of the device,thus making the device more resistant to thermal shock.

Effect Of The Invention

With the multi-layer piezoelectric device of the present inventionhaving the constitution described above, the multi-layer piezoelectricdevice having excellent durability that can be operated continuouslyover a long period of time under a high voltage and a high pressure canbe provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view showing the constitution of a multi-layerpiezoelectric device of the present invention.

FIG. 1B is an exploded perspective view showing a part of FIG. 1A.

FIG. 2 is a perspective view of the multi-layer piezoelectric device ofthe prior art.

FIG. 3 is a perspective view of an injection apparatus according to thepresent invention.

FIG. 4A is a perspective view showing the constitution of a multi-layerpiezoelectric device according to tenth embodiment of the presentinvention.

FIG. 4B is a sectional view taken along lines A-A′ of FIG. 4A.

[Description of Reference Numerals]  1, 11: Piezoelectric material  1a,13: Stack  2, 12: Internal electrode  3: Insulating material  5: Leadwire  4, 15: External electrode  6, 14: Inactive region  7: Electricalconductivity assisting member 31: Container 33: Injection hole 35: Valve43: Piezoelectric actuator

BEST MODE FOR CARRYING OUT THE INVENTION

The multi-layer piezoelectric device according to the embodiments of thepresent invention will be described in detail below.

FIG. 1A is a perspective view showing the constitution of themulti-layer piezoelectric device according to the embodiments of thepresent invention. FIG. 1B is an exploded perspective view showing apart of FIG. 1A, depicting the constitution of stacking piezoelectriclayers 11 and internal electrode layers 12.

In the multi-layer piezoelectric device according to the embodiments ofthe present invention, end of the internal electrode 12 is connected tothe external electrode 15 in every other layer on a pair of side facesof a stack 13 that is constituted by stacking the piezoelectric layers11 and the internal electrode layers 12 alternately one on another asshown in FIGS. 1A and 1B.

The stack is formed in such a constitution as (1) one of the twoadjacent internal electrodes has one end thereof that is exposed on oneside face where the external electrode is formed, with the otherinternal electrode is located inside so that the end thereof is notexposed on one side face, and (2) one of the two adjacent internalelectrodes is located inside so that the end thereof is not exposed onthe other side face where the external electrode is formed, with theother internal electrode having the end thereof being exposed on theother end face, while the external electrodes 15 are formed on one sideface and on the other side face of the stack. With this constitution,the end of the internal electrode 12 is connected to the externalelectrode 15 in every other layer on the side faces where the externalelectrodes are formed.

On both ends of the stack 13 in the direction of stacking, inactivelayers are formed whereon only the piezoelectric layers 11 are formedwithout the internal electrode layers. When the multi-layerpiezoelectric device of this embodiment is used as the multi-layerpiezoelectric actuator, lead wires may be connected to the externalelectrodes 15 by soldering, with the lead wires being connected to apower source installed outside.

In the multi-layer piezoelectric device according to the embodimenthaving the constitution described above, a predetermined voltage isapplied to the piezoelectric layers 11 via the internal electrode 12, sothat the piezoelectric layers 11 undergo a displacement by the reversepiezoelectric effect.

The inactive layers 14, in contrast, does not undergo a displacementeven when a voltage is applied, since they are a plurality of layers ofthe piezoelectric layer 11 where the internal electrodes 12 are notprovided.

Now embodiments of the present invention will be described in detailbelow.

FIRST EMBODIMENT

In the multi-layer piezoelectric device according to the firstembodiment of the present invention, concentration of alkali metal inthe piezoelectric layer 11 is in a range from 5 ppm to 500 ppm. When theconcentration of alkali metal is lower than 5 ppm, the effect ofassisting the sintering reaction is significantly low and thepiezoelectric layer cannot be sintered unless the firing temperature israised. Therefore, such low concentration of alkali metal is undesirablesince the metal constituting the internal electrode is melted whenforming the stack 13 at a high temperature. When the concentration ishigher than 500 ppm, specific resistance of the device changes when themulti-layer piezoelectric device operated continuously, thus leading toa change in the amount of displacement and malfunction of the device.

In order to minimize the change in the amount of displacement duringcontinuous operation, concentration of alkali metal in the piezoelectriclayer 11 is preferably set in a range from 5 ppm to 100 ppm.

In order to further decrease the change in the amount of displacement,concentration of alkali metal in the piezoelectric layer 11 is morepreferably set in a range from 5 ppm to 50 ppm.

Now a method for controlling the concentration of alkali metal in thepiezoelectric layer within the above range will be described below.

In order to control the concentration of alkali metal in thepiezoelectric layer 11, alkali metal may be added in the form ofcompound such as oxide, carbonate or nitrate as an inevitable impurityto the stock material of the piezoelectric layer 11 and the bindermaterial, but the method is not limited to this. In case alkali metalcontent in the multi-layer piezoelectric device is controlled to copewith the alkali metal that mixes in from the crushing balls used incrushing and mixing of the stock material of the piezoelectric layer 11and the impurity that diffuses into the material from the firingatmosphere, such a manufacturing method may also be employed as themanufacturing process is separated from that of other product so as toprevent alkali metals from mixing in from the other manufacturingprocess, and controlled quantities of alkali metal oxide, alkali metalcarbonate or alkali metal nitrate are added as inevitable impurities tothe stock material.

SECOND EMBODIMENT

The multi-layer piezoelectric device according to the second embodimentof the present invention contains alkali metal in the internalelectrodes 12 in a concentration from 5 ppm to 500 ppm. When theconcentration of alkali metal is lower than 5 ppm, the effect ofassisting the sintering reaction is significantly low and the internalelectrodes 12 cannot be sintered unless the firing temperature israised. Therefore, such low concentration of alkali metal isundesirable. When the concentration is higher than 500 ppm, alkali metalions diffuse from the internal electrodes 12 serving as the positiveelectrode into the piezoelectric layer 11 when a high DC voltage isapplied to the multi-layer piezoelectric device, thereby decreasing theresistance of the piezoelectric layer 11. As a result, specificresistance of the device changes when the multi-layer piezoelectricdevice is operated continuously, thus leading to a change in the amountof displacement and malfunction of the device.

In order to minimize the change in the amount of displacement duringcontinuous operation, concentration of alkali metal in the internalelectrodes is preferably set in a range from 5 ppm to 100 ppm.

In order to further decrease the change in the amount of displacement,concentration of alkali metal in the internal electrodes is morepreferably set in a range from 5 ppm to 50 ppm.

Now a method for controlling the concentration of alkali metal in theinternal electrodes within the above range will be described below.

In order to control the concentration of alkali metal in the internalelectrodes 12, alkali metal may be added in the form of compound such asoxide, carbonate or nitrate as an inevitable impurity to the stockmaterial of the internal electrodes 12 and the binder material, but themethod is not limited to this. In case alkali metal content in themulti-layer piezoelectric device is controlled to cope with the alkalimetal that mixes in from the crushing balls used in crushing and mixingof the stock material of the internal electrodes 12 and the impuritythat diffuses into the material from the firing atmosphere, such amanufacturing method may also be employed as the manufacturing processis separated from that of other product so as to prevent alkali metalsfrom mixing in from the other manufacturing process, and controlledquantities of alkali metal oxide, alkali metal carbonate or alkali metalnitrate are added as inevitable impurities to the stock material.

THIRD EMBODIMENT

The multi-layer piezoelectric device according to the third embodimentof the present invention contains alkali metal in the externalelectrodes 15 in a concentration from 5 ppm to 500 ppm. When theconcentration of alkali metal is lower than 5 ppm, the effect ofassisting the sintering reaction is significantly low and the externalelectrodes 15 cannot be sintered unless the firing temperature israised. Therefore, such low concentration of alkali metal is undesirablesince the metal constituting the internal electrode 12 is melted whenfired to such a high temperature. Furthermore, adhesion to thepiezoelectric layer 11 decreases, and it is not preferred. When theconcentration is higher than 500 ppm, alkali metal ions diffuse from theexternal electrodes 15 serving as the positive electrode into thepiezoelectric layer 11 when a high DC voltage is applied to themulti-layer piezoelectric device, thereby decreasing the resistance ofthe piezoelectric layer 11. As a result, specific resistance of thedevice changes when the multi-layer piezoelectric device operatedcontinuously, thus leading to a change in the amount of displacement andmalfunction of the device.

In order to minimize the change in the amount of displacement duringcontinuous operation, concentration of alkali metal in the externalelectrodes is preferably set in a range from 5 ppm to 100 ppm.

In order to further decrease the change in the amount of displacement,concentration of alkali metal in the external electrodes is morepreferably set in a range from 5 ppm to 50 ppm.

Now a method for controlling the concentration of alkali metal in theexternal electrodes within the above range will be described below.

In order to control the concentration of alkali metal in the externalelectrodes 15, alkali metal may be added in the form of compound such asoxide, carbonate or nitrate as an inevitable impurity to the stockmaterial of the external electrodes 15 and the binder material, but themethod is not limited to this. In case alkali metal content in themulti-layer piezoelectric device is controlled to cope with the alkalimetal that mixes in from the crushing balls used in crushing and mixingof the stock material of the external electrodes 15 and the impuritythat diffuses into the material from the firing atmosphere, such amanufacturing method may also be employed as the manufacturing processis separated from that of other product so as to prevent alkali metalsfrom mixing in from the other manufacturing process, and controlledquantities of alkali metal oxide, alkali metal carbonate or alkali metalnitrate are added as inevitable impurities to the stock material.

FOURTH EMBODIMENT

The multi-layer piezoelectric device according to the fourth embodimentof the present invention contains alkali metal in the multi-layerpiezoelectric device in a concentration from 5 ppm to 300 ppm. When theconcentration of alkali metal is lower than 5 ppm, the effect ofassisting the sintering reaction is significantly low and the stack 13cannot be sintered unless the firing temperature is raised. Therefore,such low concentration of alkali metal is undesirable. When theconcentration is higher than 300 ppm, specific resistance of the devicechanges when the multi-layer piezoelectric device is operatedcontinuously, thereby causing the amount of displacement to change andcausing malfunction of the device to occur. In order to minimize thechange in the amount of displacement of the multi-layer piezoelectricdevice during continuous operation, concentration of alkali metal in themulti-layer piezoelectric device is preferably set in a range from 5 ppmto 100 ppm, and more preferably in a range from 5 ppm to 50 ppm.

Now a method for controlling the concentration of alkali metal in themulti-layer piezoelectric device within the above range will bedescribed below.

In order to control the concentration of alkali metal in the multi-layerpiezoelectric device, alkali metal may be added in the form of compoundsuch as oxide, carbonate or nitrate as an inevitable impurity to thestock materials of the piezoelectric layers 11, the internal electrodelayers 12, the external electrodes 15 and the binder material, but themethod is not limited to this. In case alkali metal content in themulti-layer piezoelectric device is controlled to cope with the alkalimetal that mixes in from the crushing balls used in crushing and mixingof the stock material of the piezoelectric layer 11 and the impuritythat diffuses into the material from the firing atmosphere, such amanufacturing method may also be employed as the manufacturing processis separated from that of other product so as to prevent alkali metalsfrom mixing in from the other manufacturing process, and controlledquantities of alkali metal oxide, alkali metal carbonate or alkali metalnitrate that become the inevitable impurities are added to the stockmaterial.

In the first through fourth embodiments, alkali metal contents in thepiezoelectric layers 11, in the internal electrode layers 12 and in theexternal electrodes can be measured by applying ICP emissionspectrochemical analysis to a cut surface of the multi-layerpiezoelectric device that has been processed so as to selectively leavethe desired portions to remain through etching or the like therebyseparating the piezoelectric layers, the internal electrode layers andthe external electrodes. Alkali metal contents in the multi-layerpiezoelectric device can be measured by applying ICP emissionspectrochemical analysis to a sample of the multi-layer piezoelectricdevice. The method is not limited to the ICP emission spectrochemicalanalysis, and Auger electron spectroscopy, EPMA (Electron Probe MicroAnalysis) or the like may be used as long as the minimum detectablelimit is at similar level.

According to the present invention, the alkali metal is preferably atleast one kind of Na and K. While alkali metals include lithium, sodium,potassium, rubidium, cesium and francium, Na and K have higher tendencyto ionize and migrate and therefore are advantageously used indecreasing the device resistance of the multi-layer piezoelectric devicethereby decreasing the amount of displacement of the multi-layerpiezoelectric device.

FIFTH EMBODIMENT

The multi-layer piezoelectric device according to the fifth embodimentof the present invention contains halogen element in the piezoelectriclayer in a concentration from 5 ppm to 1500 ppm. When the concentrationof halogen element is lower than 5 ppm, the effect of assisting thesintering reaction is significantly low and the piezoelectric layerscannot be sintered unless the firing temperature is raised. Therefore,such low concentration of halogen element is undesirable since the metalconstituting the internal electrode is melted when the stack 13 isformed. When the concentration is higher than 1500 ppm, specificresistance of the device changes when the multi-layer piezoelectricdevice operated continuously, thus leading to a change in the amount ofdisplacement which leads to malfunction of the device orshort-circuiting that interrupts the operation.

In order to minimize the change in the amount of displacement of thedevice during continuous operation, concentration of halogen element inthe piezoelectric layers is preferably set in a range from 5 ppm to 100ppm.

In order to further decrease the change in the amount of displacement ofthe device, concentration of halogen element in the piezoelectric layersis more preferably set in a range from 5 ppm to 50 ppm.

Now a method for controlling the concentration of halogen element in thepiezoelectric layers within the above range will be described below.

In order to control the concentration of halogen element in thepiezoelectric layers 11, halogen element may be added in the form ofcompound such as fluoride, chloride, bromide, iodide or astatinecompound as an inevitable impurity to the stock material of thepiezoelectric layers 11 and the binder material, but the method is notlimited to this. In case halogen element content in the multi-layerpiezoelectric device is controlled so as to cope with the halogenelement that mixes in from the crushing balls used in crushing andmixing of the stock material of the piezoelectric layers 11 and theimpurity that diffuses into the material from the firing atmosphere,such a manufacturing method may also be employed as the manufacturingprocess is separated from that of other product so as to prevent halogenelement from mixing in from the other manufacturing process, andcontents of fluoride, chloride, bromide, iodide or astatine compoundthat become the inevitable impurities of halogen component in the stockmaterial are controlled, thereby controlling the halogen elementcontent.

SIXTH EMBODIMENT

The multi-layer piezoelectric device according to the sixth embodimentof the present invention contains halogen element in the internalelectrodes in a concentration from 5 ppm to 1500 ppm. When theconcentration of halogen element is lower than 5 ppm, the effect ofassisting the sintering reaction is significantly low and the internalelectrodes cannot be sintered unless the firing temperature is raised.Therefore, such low concentration of halogen element is undesirable.When the concentration is higher than 1500 ppm, halogen ions diffusefrom the internal electrodes 12 serving as the negative electrode intothe piezoelectric layer 11 when a high DC voltage is applied to themulti-layer piezoelectric device, thereby decreasing the resistance ofthe piezoelectric layer 11. As a result, specific resistance of thedevice changes, thus leading to a change in the amount of displacementand malfunction of the device.

In order to minimize the change in the amount of displacement duringcontinuous operation, concentration of halogen element in the internalelectrodes is preferably set in a range from 5 ppm to 100 ppm.

In order to further decrease the change in the amount of displacement,concentration of halogen element in the internal electrodes is morepreferably set in a range from 5 ppm to 50 ppm.

Now a method for controlling the concentration of halogen element in theinternal electrodes within the above range will be described below.

In order to control the concentration of halogen element in the internalelectrodes 12, halogen element may be added in the form of compound suchas fluoride, chloride, bromide, iodide or astatine compound asinevitable impurity to the stock material of the internal electrodes 12and the binder material, but the method is not limited to this. In casehalogen element content in the multi-layer piezoelectric device iscontrolled so as to cope with the halogen element that mixes in from thecrushing balls used in crushing and mixing of the stock material of thepiezoelectric layers 11 and the impurity that diffuses into the materialfrom the firing atmosphere, such a manufacturing method may also beemployed as the manufacturing process is separated from that of otherproduct so as to prevent halogen element from mixing in from the othermanufacturing process, and the contents of fluoride, chloride, bromide,iodide or astatine compound that become the inevitable impurities of thestock material are controlled, thereby controlling the halogen elementcontent.

SEVENTH EMBODIMENT

The multi-layer piezoelectric device according to the seventh embodimentof the present invention contains alkali metal in the externalelectrodes in a concentration from 5 ppm to 500 ppm. When theconcentration of alkali metal is lower than 5 ppm, the effect ofassisting the sintering reaction is significantly low and the externalelectrodes cannot be sintered unless the firing temperature is raised.Therefore, such low concentration of alkali metal is undesirable sincethe metal of the internal electrode is melted at such a high temperaturewhen fired. When the concentration is higher than 500 ppm, halogen ionsdiffuse from the external electrodes 15 serving as the negativeelectrode into the piezoelectric layer 11 when a high DC voltage isapplied to the multi-layer piezoelectric device, thereby decreasing theresistance of the piezoelectric layer 11. As a result, specificresistance of the device changes, thus leading to a change in the amountof displacement and malfunction of the device.

In order to minimize the change in the amount of displacement of thedevice during continuous operation, concentration of halogen element inthe external electrodes is preferably set in a range from 5 ppm to 100ppm.

In order to further decrease the change in the amount of displacement ofthe device, concentration of halogen element in the external electrodesis more preferably set in a range from 5 ppm to 50 ppm.

Now a method for controlling the concentration of halogen element in theexternal electrodes within the above range will be described below.

In order to control the concentration of halogen element in the externalelectrodes 15, halogen element may be added in the form of compound suchas fluoride, chloride, bromide, iodide or astatine compound asinevitable impurity to the stock material of the external electrodes 15and the binder material, but the method is not limited to this. In casehalogen element content in the multi-layer piezoelectric device iscontrolled so as to cope with the halogen element that mixes in from thecrushing balls used in crushing and mixing of the stock material of thepiezoelectric layers 11 and the impurity that diffuses into the materialfrom the firing atmosphere, such a manufacturing method may also beemployed as the manufacturing process is separated from that of otherproduct so as to prevent halogen element from mixing in from the othermanufacturing process, and the contents of fluoride, chloride, bromide,iodide or astatine compound that become the inevitable impurities of thestock material are controlled, thereby controlling the halogen elementcontent.

EIGHTH EMBODIMENT

The multi-layer piezoelectric device according to the eighth embodimentof the present invention contains halogen element in the multi-layerpiezoelectric device in a concentration from 5 ppm to 300 ppm. When theconcentration of halogen element is lower than 5 ppm, the effect ofassisting the sintering reaction is significantly low and the stack 13cannot be sintered unless the firing temperature is raised. Therefore,such low concentration of halogen element is undesirable. When theconcentration is higher than 300 ppm, specific resistance of the devicechanges when the multi-layer piezoelectric device is operatedcontinuously, thereby causing the amount of displacement to change andmalfunction of the device to occur. In order to minimize the change inthe amount of displacement of the multi-layer piezoelectric deviceduring continuous operation, concentration of halogen element in themulti-layer piezoelectric device is preferably set in a range from 5 ppmto 100 ppm, and more preferably set in a range from 5 ppm to 50 ppm.

Now a method for controlling the concentration of halogen element in themulti-layer piezoelectric device within the above range will bedescribed below. In order to control the concentration of halogenelement in the multi-layer piezoelectric device, halogen element may beadded in the form of compound such as fluoride, chloride, bromide,iodide or astatine compound as inevitable impurity to the stockmaterials of the piezoelectric layers 11, the internal electrode layers12, the external electrodes 15 and the binder material, but the methodis not limited to this. In case halogen element content in themulti-layer piezoelectric device is controlled to cope with the halogenelement that mixes in from the crushing balls used in crushing andmixing of the stock material of the piezoelectric layers 11 and theimpurity that diffuses into the material from the firing atmosphere,such a manufacturing method may also be employed as the manufacturingprocess is separated from that of other product so as to prevent halogenelement from mixing in from the other manufacturing process, and thecontents of fluoride, chloride, bromide, iodide or astatine compoundthat become the inevitable impurities of the stock material arecontrolled, thereby controlling the halogen element content.

In the fifth through eighth embodiments, halogen element contents in thepiezoelectric layers, in the internal electrode layers and in theexternal electrodes can be measured by ion chromatography with a cutsurface of the multi-layer piezoelectric device that has been processedso as to selectively leave the desired portions to remain throughetching or the like thereby separating the piezoelectric layers, theinternal electrode layers and the external electrodes. Halogen elementcontent in the multi-layer piezoelectric device can be measured byapplying the ion chromatography method to a sample of the multi-layerpiezoelectric device. The method is not limited to the ionchromatography, and Auger electron spectroscopy, EPMA (Electron ProbeMicro Analysis) or the like may be used as long as the minimumdetectable limit is at similar level.

More preferably, the multi-layer piezoelectric device may include alkalimetal in a concentration from 5 ppm to 300 ppm and halogen element in aconcentration from 5 ppm to 1000 ppm. This makes it possible to preventthe metal elements that constitute the electrodes from diffusing in sucha process as silver migration, in addition to the effects describedabove.

According to the present invention, the halogen element is preferably atleast one kind of Cl and Br. While halogen elements include fluorine,chlorine, bromine, iodine and astatine, Cl and Br have higher tendencyto ionize and migrate and therefore are advantageously used indecreasing the device resistance of the multi-layer piezoelectric devicethereby decreasing the amount of displacement of the multi-layerpiezoelectric device.

By controlling the concentrations of alkali metal and halogen element inthe multi-layer piezoelectric device of the present invention within theranges described above, it is made possible to keep the heat generatedduring the continuous operation constant and control the amount ofdisplacement constant.

In order to suppress the heat generated by the multi-layer piezoelectricdevice during operation, it is necessary to decrease the induction loss(tan δ) of the piezoelectric layer 11 and/or device resistance, inaddition to controlling the impurity concentrations within the rangesdescribed above.

NINTH EMBODIMENT

In the multi-layer piezoelectric device according to the ninthembodiment of the present invention, the ratio of change in thedimension of the multi-layer piezoelectric device after continuousoperation to the initial device dimension is set within 1%. This isbecause the ratio of change in the dimension of the multi-layerpiezoelectric device after continuous operation higher than 1% leads toa great change in the dimension of the multi-layer piezoelectric devicewhich may cause destruction of the multi-layer piezoelectric device dueto thermal excursion.

The ratio of change in device dimension after continuous operation tothe initial dimension refers to the ratio of change in the dimension ofthe multi-layer piezoelectric device in the stacking direction, afterabout 1×10⁹ cycles of continuous operation under an AC voltage appliedto the multi-layer piezoelectric device, to the initial device dimensionof the multi-layer piezoelectric device before the continuous operation.

In the multi-layer piezoelectric device of the present invention, theratio of change in thickness of the internal electrode 12 aftercontinuous operation of the multi-layer piezoelectric device to theinitial thickness is set within 5%. This is because, when the ratio ofchange in thickness of the internal electrode 12 after continuousoperation of the multi-layer piezoelectric device to the initialthickness is higher than 5%, significant deterioration of themulti-layer piezoelectric device that is represented by the change inthe amount of displacement of the multi-layer piezoelectric deviceoccurs, thus resulting in a significant deterioration of durability ofthe multi-layer piezoelectric device.

The ratio of change in thickness of the internal electrode aftercontinuous operation to the initial thickness refers to the ratio ofchange in the thickness of the internal electrode in the stackingdirection of the multi-layer piezoelectric device, after about 1×10⁹cycles of continuous operation under an AC voltage applied to themulti-layer piezoelectric device, to the initial thickness of theinternal electrode before the continuous operation. In case five or moreinternal electrodes 12 are provided in the multi-layer piezoelectricdevice, thickness is measured by means of SEM for the internalelectrodes 12 making contact with the inactive layers 14 (at twopositions), the internal electrode 12 located at intermediate positionin the stacking direction of the multi-layer piezoelectric device (atone position) and any of the internal electrodes 12 located atintermediate position between the intermediately positioned internalelectrodes 12 and the inactive layer 14 (at one position), and the meanvalue of these measurements is taken as the thickness of the internalelectrode. In case less than five internal electrodes 12 are provided inthe multi-layer piezoelectric device, thickness is measured on everyinternal electrode 12, and the mean value of these measurements is takenas the thickness of the internal electrode.

In the multi-layer piezoelectric device according to the ninthembodiment of the present invention, it is necessary to preventvolumetric expansion of the internal electrodes 12 from occurring due tooxidation, in order to suppress the device dimension and thickness ofthe internal electrodes 12 from changing after continuous operation.Volumetric expansion of the internal electrodes 12 can be suppressed asfollows.

In the prior art, such means have been employed as to keep the devicetemperature constant during continuous operation or finely adjust theapplied voltage in accordance to the device temperature, in order tosuppress the device dimension and thickness of the internal electrodesfrom changing after continuous operation. Specifically, the operatingvoltage has controlled while monitoring the device temperature, or aheat sink has been installed in order to dissipate heat and control thetemperature around the device.

In the ninth embodiment, in contrast, the device temperature iscontrolled during continuous operation by suppressing the heatgeneration from the device at the source during operation. In order tocontrol the device temperature during operation, it is necessary todecrease the induction loss (tan δ) of the piezoelectric layer 11 and/ordecrease device resistance

In case the piezoelectric layer 11 is constituted from perovskite typeoxide such as PbZrO₃—PbTiO₃ as the main component, induction loss (tanδ) of the piezoelectric layer 11 may be reduced by firing the stack 13in an atmosphere of excessive oxygen, or cooling down at a slow ratefrom the peak temperature of firing the stack 13. Specifically, coolingrate may be set to 600° C. per hour or less, or preferably 300° C. perhour or less. Value of induction loss (tan δ) may be less than 1.5%, orpreferably 0.5% or less.

In order to decrease the device resistance, a dense structure may beformed with a path for electrical conductivity secured therein, inaddition to using a material of a composition having lower specificresistance for the internal electrode 12.

Moreover, since it is desirable that the amount of displacement of thematerial constituting the piezoelectric layer 11 is constant regardlessof the operating temperature, a piezoelectric material that undergoessmaller change in the amount of displacement as the temperature variesduring continuous operation.

In order to dissipate heat efficiently from the device to the outside,it is preferable to form the internal electrode 12 through which heat istransmitted from a material having composition of excellent heatconductivity.

Moreover, it is desirable that the metal compound that constitutes theinternal electrode 12 contains group VIII metal and/or group Ib metal asthe main component. Since the metals of these groups have high heatresistance, the internal electrode 12 can be fired together with thepiezoelectric layer 11 that has high firing temperature at the sametime.

It is also preferable that the metal compound that constitutes theinternal electrode 12 has proportion M1 (% by weight) of the group VIIImetal and proportion M2 (% by weight) of the group Ib metal that satisfythe relations 0<M1≦15, 85≦M2<100 and M1+M2=100. This is becauseproportion of group VIII metal higher than 15% by weight leads to highspecific resistance of the internal electrode 12 which causes theinternal electrode 12 to generate heat when the multi-layerpiezoelectric device is operated continuously. In order to suppress thegroup Ib metal contained in the internal electrode 12 from migratinginto the piezoelectric layer 11, proportion of group VIII metal ispreferably in a range from 0.001% by weight to 15% by weight. In orderto improve the durability of the multi-layer piezoelectric device, theproportion is preferably in a range from 0.1% by weight to 10% byweight. When higher heat conductivity and higher durability arerequired, the proportion is preferably in a range from 0.5% by weight to9.5% by weight. In order to improve durability further, the proportionis preferably in a range from 2% by weight to 8% by weight.

When the proportion of Ib metal is less than 85% by weight, the internalelectrode 12 has high specific resistance that causes the internalelectrode 12 to generate heat when the multi-layer piezoelectric deviceis operated continuously. In order to suppress the group Ib metalcontained in the internal electrode 12 from migrating into thepiezoelectric layer 11, proportion of group Ib metal is preferably in arange from 85% by weight to 99.999% by weight. In order to improve thedurability of the multi-layer piezoelectric device, the proportion ispreferably in a range from 90% by weight to 99.9% by weight. When higherdurability is required, the proportion is preferably in a range from90.5% by weight to 99.5% by weight. In order to improve durabilityfurther higher, the proportion is preferably in a range from 92% byweight to 98% by weight.

The group VIII metal and the group Ib metal in the internal electrode 12can be identified by analytical method such as EPMA (Electron ProbeMicro Analysis).

Metal elements contained in the internal electrode 12 of the presentinvention are preferably the group VIII metal that is at least one kindselected from among Ni, Pt, Pd, Rh, Ir, Ru and Os, and the group Ibmetal that is at least one kind selected from among Cu, Ag and Au, sincesuch a metal composition is advantageous in volume production when thealloy powder synthesizing technology available today is employed.

Further, it is preferable that the group VIII metal contained in theinternal electrode 12 is at least one kind selected from among Pt andPd, and the group Ib metal is at least one kind selected from among Agand Au. This compositions makes it possible to form the internalelectrode 12 having high heat resistance and low specific resistance.

Further, it is preferable that the group VIII metal contained in theinternal electrode 12 is Ni and the group Ib metal is Cu. Thiscompositions makes it possible to form the internal electrode 12 havinghigher durability and excellent heat conductivity.

Further it is preferable to add an inorganic component along with themetallic component in the internal electrode 12. This increases thebonding strength between the internal electrode 12 and the piezoelectriclayer 11. It is preferable that the inorganic component containsperovskite type oxide consisting of PbZrO₃—PbTiO₃ as the main component.

It is further preferable that the piezoelectric layer 11 containsperovskite type oxide as the main component. This is because thepiezoelectric layer 11 formed from perovskite type oxide such as bariumtitanate (BaTiO₃) has high piezoelectric strain constant d₃₃ whichenables it to increase the amount of displacement. This constitutionalso enables the piezoelectric layer 11 and the internal electrode 12 tobe fired at the same time. It is also preferable that the piezoelectriclayer 11 contains perovskite type oxide consisting of PbZrO₃—PbTiO₃ thathas a relatively high value of piezoelectric strain constant d₃₃ as themain component.

The firing temperature is preferably in a range from 900 to 1000° C.When the firing temperature is lower than 900° C., the firing processdoes not fully proceed, and it becomes difficult to make densepiezoelectric layer 11. When the firing temperature is higher than 1000°C., larger stress is generated due to the difference in contractionbetween the internal electrode 12 and the piezoelectric layer 11 whenfired, thus resulting in cracks occurring during continuous operation ofthe multi-layer piezoelectric device.

The deviation in the composition of the internal electrode 12 that iscaused by the firing operation is preferably not larger than 5%. This isbecause a deviation larger than 5% in the composition of the internalelectrode 12 caused by the firing operation causes a greater amount ofthe metallic component contained in the internal electrode 12 to diffuseinto the piezoelectric layer 11, thus making it impossible for theinternal electrode 12 to deform in conformity with the expansion andcontraction of the multi-layer piezoelectric device during operation.

The deviation in the composition of the internal electrode 12 refers tothe variation in the composition of the internal electrode 12 caused byevaporation of the elements that constitute the internal electrode 12due to firing or diffusion thereof into the piezoelectric layer 11.

In the multi-layer piezoelectric device of the present invention, theinternal electrode 12 of which end is exposed on the side face of themulti-layer piezoelectric device and the internal electrode 12 of whichend is not exposed are stacked alternately, while a groove is formed inthe piezoelectric layer located between the internal electrode 12 ofwhich end is not exposed and the external electrode 15. The groove ispreferably filled with an insulating material having Young's moduluslower than that of the piezoelectric layer 11. In the multi-layerpiezoelectric device having the groove filled with an insulatingmaterial having low Young's modulus, stress caused by the displacementduring operation can be mitigated, thus enabling it to suppress heatgeneration from the internal electrode 12 even when operatedcontinuously.

The multi-layer piezoelectric devices of the first through ninthembodiments are manufactured as described below.

First, a calcined powder of a piezoelectric ceramic material constitutedfrom perovskite type oxide consisting of PbZrO₃—PbTiO₃ or the like, abinder made of an organic polymer such as acrylic resin or butyral resinand a plasticizer such as DOP (dioctyl phthalate) or DBP (dibutylphthalate) are mixed to form a slurry which is formed into a ceramicgreen sheet that would become the piezoelectric layer 11 by a knownmethod such as doctor blade process or calender roll process or othertape molding method.

Then a metal powder such as silver-palladium that constitutes theinternal electrode, a binder and a plasticizer are mixed to prepare anelectrically conductive paste which is applied onto the top surface ofthe ceramic green sheet by screen printing or the like to a thickness of1 to 40 μm.

A plurality of the green sheets having the electrically conductive pasteprinted thereon are stacked one on another, with the stack being heatedat a predetermined temperature to remove the binder. The stack is thenfired at a temperature in a range from 900 to 1200° C. thereby to makethe stack 13. Firing temperature is preferably in a range from 900 to1000° C.

The method of making the stack 13 is not limited to that describedabove, and any manufacturing method may be employed as long as the stack13 can be made in such a constitution as a plurality of thepiezoelectric layers 11 external electrode forming surface of thecolumn-like stack 13, and is bonded by baking at a temperature that ishigher than the softening point of the glass and is not higher than themelting point (965° C.) of silver and is not higher than ⅘ of the firingtemperature (0° C.). In this process, the binder contained in the sheetthat is formed from the electrically conductive silver-glass paste isevaporated and removed, so that the external electrode 15 is formed froma porous electrical conductor having three-dimensional mesh structure.

The temperature at which the electrically conductive silver-glass pasteis bonded by baking is preferably in a range from 550 to 700° C. for thepurpose of effectively forming a neck, joining the silver content thatis contained in the electrically conductive silver-glass paste and theinternal electrode 12 through diffusion bonding, effectively causing thevoids in the external electrode 15 to remain and partially joining theexternal electrode 15 and the side face of the column-like stack 13.Softening point of the glass component contained in the electricallyconductive silver-glass paste is preferably in a range from 500 to 700°C.

When the baking temperature is higher than 700° C., sintering of thesilver powder of the electrically conductive silver-glass paste wouldproceed excessively, such that the porous electrical conductor ofthree-dimensional mesh structure cannot be effectively formed and theexternal electrodes 15 become too dense. As a result, the value ofYoung's modulus of the external electrode 15 becomes too high toeffectively absorb the stress generated during operation, eventuallyleading to breakage of the external electrode 15. Baking is preferablycarried out at a temperature that is not higher than 1.2 times thesoftening point of the glass.

When the baking temperature is lower than 550° C., on the other hand,the end of the internal electrode 12 and the external electrode 15cannot be joined sufficiently through diffusion, and therefore the neckcannot be formed thus resulting in spark occurring between the internalelectrode 12 and the external electrode 15 during operation.

Thickness of the sheet formed from the electrically conductivesilver-glass paste is preferably smaller than the thickness of thepiezoelectric layer 11. More preferably, the thickness is 50 μm or lessin order to accommodate the contraction and expansion of the actuator.

Then the stack 13 having the external electrodes 15 formed thereon isimmersed in a silicone rubber solution while deaerating the siliconerubber solution by evacuation, so as to fill the groove of the stack 13with the silicone rubber. Then the stack 13 is pulled out of thesilicone rubber solution and is coated with the silicone rubber on theside faces thereof. Then the silicon rubber that fills the groove andcovers the side faces of the column-like stack 13 is hardened, therebycompleting the multi-layer piezoelectric device of the presentinvention.

Then lead wires are connected to the external electrodes 15 and a DCvoltage of 0.1 to 3 kV/mm is applied between the pair of externalelectrodes 15 via the lead wires so as to apply polarization treatmentto the stack 13, thereby to complete the multi-layer piezoelectricactuator that employs the multi-layer piezoelectric device of thepresent invention. When the lead wires are connected to an externalvoltage source and the voltage is supplied via the lead wires and theexternal electrodes 15 to the internal electrodes 12, the piezoelectriclayers 11 undergo a significant amount of displacement by the reversepiezoelectric effect, so as to drive, for example, an automobile fuelinjection valve that supplies fuel to an engine.

An electrical conductivity assisting member formed from an electricallyconductive adhesive, containing a metal mesh and a plurality of theinternal electrodes 12 are stacked alternately one on another.

Then the internal electrode 12 of which end is exposed on the side faceof the multi-layer piezoelectric device and the internal electrode 12 ofwhich end is not exposed are formed alternately, while the groove isformed in the piezoelectric layer located between the internal electrode12 of which end is not exposed and the external electrode 15, with thegroove filled with an insulating material such as resin or rubber havingYoung's modulus lower than that of the piezoelectric layer 11. Thegroove is formed on the side face of the stack 13 by means of a dicingapparatus or the like.

The electrically conductive material that constitutes the externalelectrode 15 is preferably silver that has a low value of Young'smodulus or an alloy based on silver, in consideration of the capabilityto sufficiently absorb the stress generated by the expansion andcontraction of the actuator.

Then an electrically conductive silver-glass paste is prepared by addinga binder to a glass powder, and the mixture is formed into a sheet thatis dried to remove solvent while controlling the density of the greensheet in a range from 6 to 9 g/cm³. The sheet is transferred onto the ora mesh-like metal sheet embedded therein, may be provided on theexternal surface of the external electrode 15. By providing theelectrical conductivity assisting member on the external surface of theexternal electrode 15, it is made possible to cause a large current toflow in the electrical conductivity assisting member even when thedevice is operated at a high speed by supplying a large current to theactuator so as to decrease the current flowing in the externalelectrodes 15, thereby preventing the external electrodes 15 frombreaking due to localized heat generation and greatly improvingdurability. Moreover, by embedding the metal mesh or a mesh-like metalsheet in the electrically conductive adhesive, it is made possible toprevent the electrically conductive adhesive from cracking due toexpansion and contraction of the stack during operation.

The metal mesh refers to a structure of entwined metal wires, and themesh-like metal sheet refers to a metal sheet with a number of holespunched therethrough.

It is preferable that the electrically conductive adhesive thatconstitutes the electrical conductivity assisting member is polyimideresin containing silver powder dispersed therein. When polyimide resinthat has high heat resistance and contains silver powder which has lowspecific resistance dispersed therein is used as the electricallyconductive adhesive, such an electrical conductivity assisting membercan be formed that maintains low resistance and high bonding strengtheven when the stack is operated at a high temperature. More preferably,the electrically conductive particles are non-spherical particles havingsuch shapes as flakes or acicular particles. When the electricallyconductive particles are non-spherical particles such as flakes oracicular particles, the electrically conductive particles can be firmlyentwined with each other, thereby increasing the shear strength of theelectrically conductive adhesive.

The multi-layer piezoelectric device of the present invention is notlimited to the constitutions described above, and various modificationsmay be made without deviating from the spirit of the present invention.

While an example where the external electrodes 15 are formed on theopposing side faces of the stack 13 has been described above, a pair ofexternal electrodes may be formed, for example, on adjacent side facesaccording to the present invention.

TENTH EMBODIMENT

FIG. 4A is a perspective view showing the multi-layer piezoelectricdevice (multi-layer piezoelectric actuator) according to the tenthembodiment of the present invention. FIG. 4B is a sectional view takenalong lines A-A′ of FIG. 4A.

The multi-layer piezoelectric actuator according to the tenth embodimentcomprises a column-like stack 1 a having rectangular prism shape formedby stacking a plurality of piezoelectric layers 1 and a plurality ofinternal electrodes 2 alternately one on another, wherein the ends ofevery other internal electrodes 2 are covered by an insulating material3 on the side face of the stack 1 a, and an external electrode 4 formedfrom a porous electrical conductor made of an electrically conductivematerial containing silver as the main material and glass havingthree-dimensional mesh structure is joined with the end of the internalelectrode 2 that is not covered by the insulating material 3, while leadwires 5 are connected to the external electrodes 4, as shown in FIG. 4.Reference numeral 6 denotes an inactive layer.

The piezoelectric layer 1 is formed from PZT-based piezoelectric ceramicmaterial which will be described in detail later. The piezoelectricceramic material preferably has a high value of piezoelectric strainconstant d₃₃ that represents the piezoelectric characteristic thereof.

Thickness of the piezoelectric layer 1, namely the distance between theinternal electrodes 2, is preferably in a range from 50 to 250 μm. Thismakes it possible to make the actuator of smaller size and low profile,and prevent insulation breakdown of the piezoelectric layer 1 fromoccurring, even when a larger number of layers are stacked so as toachieve a greater amount of displacement of the multi-layerpiezoelectric actuator by applying a voltage.

While the internal electrodes 2 are provided between the piezoelectriclayers 1, metal compound of the internal electrodes 2 is constitutedfrom group VIII metal and group Ib metal of the periodic table. Thegroup VIII metal is at least one kind selected from among Pt and Pd, andthe group Ib metal is at least one kind selected from among Ag and Au.For example, Ag—Pd alloy may be used.

When proportion M1 (% by weight) of the group VIII metal and proportionM2 (% by weight) of the group Ib metal satisfy the relations 0<M1<15 and85<M2<100, low-cost multi-layer piezoelectric device can be providedsince the use of the expensive group Ib metal can be reduced. When thecontent of group Ib metal is reduced, melting point of the alloy becomeslower and only such a kind of porcelain that can be sintered at a lowtemperature of 1000° C. or less can be used. It becomes easier for thecrystal grains of the porcelain to grow when the porcelain contains 5ppm or higher and less than 100 ppm of Si content, allowing it to sinterat a lower temperature. Si content within this range does not adverselyaffect the piezoelectric characteristic.

Formed on the side face of the column-like stack 1 a in every otherlayer are grooves measuring 30 to 500 μm in depth and 30 to 200 μm inwidth in the stacking direction. The grooves are filled with glass,epoxy resin, polyimide resin, polyamide-imide resin, silicone rubber orthe like that has Young's modulus lower than that of the piezoelectriclayer 1 so as to form an insulating material 3. The insulating material3 is preferably a material having a low value of elastic coefficient,particularly silicone rubber or the like, that can deform in conformitywith the displacement of the stack 1 a, in order to make a firm jointwith the column-like stack 1 a.

The external electrodes 4 are connected to the two opposing side facesof the column-like stack 1 a, and the external electrodes 4 areelectrically connected to the internal electrodes 2 that are stacked inevery other layer. The external electrodes 4 serve to supply the voltagethat is required in common to cause the piezoelectric layers 1 toundergo displacement by the reverse piezoelectric effect, to theinternal electrodes 2 that are connected thereto.

Connected to the external electrode 4 are lead wires 5 by soldering. Thelead wires 5 serve to connect the external electrode 4 to an outsidepower supply.

According to the present invention, the piezoelectric ceramic materialthat constitutes the piezoelectric layers 1 is formed from PbTiO₃—PbZrO₃as the main component, and contains 5 ppm or higher and less than 100ppm of Si content. Si has an effect of improving the strength of theporcelain. When the Si content is within the range described above,segregation of Si occurs in the grain boundary so as to increase thebonding strength between crystal grains, thereby suppressing fall-off ofgrains during machining or ultrasonic cleaning. With regards to themechanism of deterioration of specific resistance, it has known thatdeterioration of the crystal grain boundary induced by DC electric fieldcorresponds to the deterioration of porcelain. In piezoelectric ceramicmaterial, crystal grain boundary has higher resistivity than the innerportion of grain has. When a DC electric field is applied, strongelectric field is generated in the crystal grain boundary due toMaxwell-Wagner type polarization. It is supposed that this electricfield causes local insulation breakdown that results in deterioration ofspecific resistance of the porcelain. Based on the above discussion, Sicontent of 100 ppm or higher leads to the formation of glass phase inthe grain boundary which increases the resistivity in the grainboundary, thus causing strong electric field in the crystal grainboundary that results in local insulation breakdown and therefore leadsto deterioration of specific resistance of the porcelain. When Sicontent is less than 100 ppm, no clear glass phase is generated in thegrain boundary and Si exists in the form of SiO₂ about the size ofsingle molecule. This results in lower resistivity in the grain boundarywhich makes local insulation breakdown less likely to occur. When Sicontent is less than 5 ppm, since the effect of improving the bondingstrength between the crystal grains becomes weaker, fall-off of grainsis likely to occur during machining or ultrasonic cleaning.

The piezoelectric ceramic material of the present invention is asintered material that is constituted substantially from perovskite typecompound except for the component constituted from Si. The phrase“constituted substantially from perovskite type compound” means that thematerial is made up of perovskite type compound except for impurities,with no other components intentionally added. The impurities (Siexcluded) may be contained in a concentration less than 100 ppm.

According to the present invention, it is preferable that Si issegregated in the crystal grain boundary and thickness of the crystalgrain boundary is 1 nm or less. When Si content is less than 100 ppm, Siis segregated in the crystal grain boundary and thickness of the crystalgrain boundary is 1 nm or less. This makes it possible to decrease thechronic change in the specific resistance while increasing the strengthof the porcelain while having no influence on the characteristics suchas piezoelectric strain constant. This is because, while solid solutionof Si in the crystal grains significantly decreases the piezoelectriccharacteristics, segregation in the crystal grain boundary mitigates theinfluence on the piezoelectric characteristics. When thickness of thecrystal grain boundary is larger than 1 nm, clear glass phase is formedin the crystal grain boundary. Presence of the glass phase in thecrystal grain boundary increases the resistivity of the grain boundaryand therefore makes microscopic insulation breakdown likely to occur inthe crystal grain boundary. Therefore, it is preferable that Si existsin the crystal grain boundary in the form of individual molecules ratherthan glass phase.

In order to increase the mechanical strength, it is preferable that meancrystal grain size A of the piezoelectric ceramic material is in a rangefrom 0.5 to 5 μm and standard deviation B of the grain sizes satisfiesthe condition that B/A is not larger than 0.5, mean void size C of thepiezoelectric ceramic material is in a range from 0.5 to 5 μm andstandard deviation D of the void sizes satisfies the condition that D/Cis not larger than 0.25, while the void ratio is 5% or less. When theseconditions are satisfied, the material becomes stronger to impactapplied from the outside and to fatigue. Destruction of piezoelectricceramic material due to rapid increase of leakage current known as theavalanche destruction is caused by structural defects such as cracks orvoids. When the porcelain has uniform microstructure, it makes anactuator, for example, that has high reliability because chronic changein the volumetric specific resistance is made smaller during continuousoperation.

When the grain size increases, the porcelain tends to have lowerstrength to breakage. When the mean crystal grain size A of thepiezoelectric ceramic material exceeds 5 μm, it becomes more likely tobreak due to impact applied from the outside or fatigue and deterioratein specific resistance, thus resulting in lower reliability. It isdifficult, on the other hand, to control the mean crystal grain size Aof the porcelain below 0.5 μm, because of the problems related to themanufacturing process such as preparation of stock material havingsmaller particle size and the firing temperature. Accordingly, that meancrystal grain size of the porcelain is preferably in a range from 1 to 3μm.

When the ratio B/A of standard deviation B to the mean crystal grainsize A of the porcelain is larger than 0.5, the porcelain cannot haveuniform microstructure due to large defects and large crystal grainscontained therein. Thus it becomes more likely to break due to impactapplied from the outside or fatigue and deteriorate in specificresistance, thus resulting in lower reliability.

Since mean crystal grain size of the porcelain is in a range from 0.5 to5 μm, mean void size C in the porcelain is also in a range from 0.5 to 5μm. As the defects become smaller, the material becomes less likely tobreak due to impact applied from the outside or fatigue and deterioratein specific resistance, thus resulting in higher reliability. When themean void size exceeds 5 μm, it becomes more likely to break due toimpact applied from the outside or fatigue and deteriorate in specificresistance, thus resulting in lower reliability. It is difficult, on theother hand, to control the mean void size below 0.5 μm, because of theproblems related to the manufacturing process such as preparation ofstock material having smaller particle size and the firing temperature.

When the ratio D/C of standard deviation D to the mean void size C ofthe porcelain is larger than 0.25, the porcelain contains large defectsformed therein, and therefore it becomes more likely to break due toimpact applied from the outside or fatigue and deteriorate in specificresistance, thus resulting in lower reliability. When the void ratioexceeds 5%, the porcelain becomes more likely to break due to impactapplied from the outside or fatigue and deteriorate in specificresistance, thus resulting in lower reliability. It is desirable thatthere exist no voids to obtain better characteristics and reliability,although it is difficult to achieve in the practical manufacturingprocess.

A method for manufacturing the multi-layer piezoelectric deviceaccording to the present invention will be described below. First,column-like stack 1 a is fabricated. Stock material of PZT is preparedby mixing predetermined quantities of high purity powders of PbO, ZrO₂,TiO₂, ZnO, Nb₂O₅, WO₃, BaCO₃, SrCO₃, Yb₂O₃ and SiO₂ in wet process bymeans of ball mill or the like for 10 to 24 hours. Then after dewateringand drying the mixture, the mixture is calcined at a temperature in arange from 800 to 900° C. for a period of 1 to 3 hours. The calcinedmaterial is then crushed in wet process by means of ball mill or thelike so as to achieve a particle size distribution of D₅₀ in a range of0.5±0.2 μm and D₉₀ less than 0.8 μm. The calcined powder ofpiezoelectric ceramic material such as the crushed PZT or the like, abinder made of an organic polymer such as acrylic resin or butyral resinand a plasticizer such as DBP (dibutyl phthalate) or DOP (dioctylphthalate) are mixed to form a slurry which is formed into a ceramicgreen sheet that would become the piezoelectric layer 1 by a knownmethod such as doctor blade process or calender roll process or othertape molding method.

Then an Ag—Pd or Pt powder, a binder and a plasticizer are mixed toprepare an electrically conductive paste which is applied onto the topsurface of the ceramic green sheet by screen printing or the like to athickness of 1 to 40 μm.

A plurality of the green sheets having the electrically conductive pasteprinted thereon are stacked one on another, with the stack being heatedat a predetermined temperature to remove the binder. Then the stack isfired at a temperature in a range from 900 to 1200° C. thereby to makethe column-like stack 1 a.

The method of making the column-like stack 1 a is not limited to thatdescribed above, and any manufacturing method may be employed as long asthe column-like stack 1 a can be made in such a constitution as aplurality of the piezoelectric layers and a plurality of the internalelectrodes are stacked alternately one on another.

Then a groove is formed on the side face of the column-like stack 1 a inevery other layer by means of a dicing apparatus or the like.

Next the electrically conductive silver-glass paste is bonded by bakingat a temperature in a range from 550 to 700° C. so as to form theexternal electrode 4.

Then the column-like stack 1 a having the external electrodes 4 formedthereon is immersed in a silicone rubber solution while deaerating thesilicone rubber solution by evacuation, so as to fill the groove of thecolumn-like stack 1 a with the silicone rubber. Then the column-likestack 1 a is pulled out of the silicone rubber solution and is coatedwith the silicone rubber on the side faces thereof. Then the siliconrubber that fills the groove and covers the side faces of thecolumn-like stack 1 a is hardened.

Then lead wires 5 are connected to the external electrodes 4, therebycompleting the multi-layer piezoelectric device of the presentinvention.

DC voltage of 0.1 to 3 kV/mm is applied between the pair of externalelectrodes 4 via the lead wires 5 so as to apply polarization treatmentto the column-like stack 1 a, thereby to complete the multi-layerpiezoelectric actuator. When the lead wires 5 are connected to anexternal voltage source and the voltage is supplied via the lead wires 5and the external electrodes 4 to the internal electrodes 2, thepiezoelectric layers 1 undergo a significant amount of displacement bythe reverse piezoelectric effect, so as to drive, for example, anautomobile fuel injection valve that supplies fuel to an engine.

In the first through tenth embodiments described above, it is preferablethat the internal electrode 2 includes voids and the voids occupy 5 to70% of cross sectional area of the internal electrode 2 (this ratio willhereinafter be referred to as the void ratio).

By using the internal electrodes 2 that include voids to constitute themulti-layer piezoelectric device, it is made possible to obtain themulti-layer piezoelectric device having high durability. When the voidratio in the internal electrodes 2 is less than 5%, greater restrictiveforce is exerted on the displacement of the piezoelectric layer, withsmaller effect of the presence of the voids. When the void ratio in theinternal electrodes 2 is more than 70%, electrical conductivity of theinternal electrodes 2 decreases and the strength decreases, which is notdesirable. In order to improve the durability of the device, void ratioin the internal electrodes 2 is preferably from 7 to 70%. Void ratio inthe internal electrodes 2 is more preferably from 10 to 60%, whichenables it to maintain a large amount of displacement and achieve highdurability.

The void ratio in the internal electrodes 2 refers to the ratio of areaoccupied by the voids to the total cross sectional area of the internalelectrode 2, as described above, and can be determined as follows.

In a longitudinal section of the multi-layer piezoelectric device cutalong a direction parallel to the stacking direction, total crosssectional area of the internal electrode 2 exposed in the longitudinalsection and the area occupied by the voids are measured, for example,under a microscope. From these areas, void ratio in the internalelectrode 2 is calculated as (area occupied by voids/total crosssectional area)×100.

The internal electrode 2 that includes the voids can be manufactured asfollows.

First, a metal powder that constitutes the internal electrode 2 isprepared from two or more kinds of materials having different meltingpoints so that voids are formed in the internal electrode 2 afterfiring. An alloy may be used as the metallic material depending on thepurpose.

The metal powder that constitutes the internal electrode 2 is calcinedat a temperature that is not lower than that of the metal having thelowest melting point and is not higher than that of the metal having thehighest melting point among the metal powder that constitutes theinternal electrode 2. When calcined at such a temperature, metal oralloy among the metal powder that constitutes the internal electrode 2that has been melted moves between metal particles that have not beenmelted due to capillary effect thus leaving voids behind. This methodallows it to set the void ratio in the internal electrode 2 to a desiredlevel by adjusting the mixing proportions and temperature of the metalpowder that constitutes the internal electrode 2.

The voids in the internal electrode 2 may also be formed by making useof small clearance that is formed between the metal powder particleswhen controlling the electrically conductive paste used in forming theinternal electrode 2, or by making use of clearance that is formed asthe binder contained in the electrically conductive paste is burned out.

Alternatively, the voids in the internal electrode 2 may also be formedby adding a material, that has low wettability with regards to thematerials that constitute the internal electrode 2, to the electricallyconductive paste used in forming the internal electrode 2, or by coatingthe green sheet of the piezoelectric material, whereon the electricallyconductive paste of the internal electrode 2 is to be printed, with amaterial that has low wettability with regards to the materials thatconstitute the internal electrode 2. For the material that has lowwettability with regards to the materials that constitute the internalelectrode 2, for example, BN can be used.

The first through tenth embodiments have been described above,exemplifying the multi-layer piezoelectric devices having specificconstitutions. According to the present invention, various multi-layerpiezoelectric devices can be constituted by combining various componentsdescribed above. For example, the device having the constitution shownin FIGS. 4A, 4B and the piezoelectric layers, the internal electrodes orthe external electrodes described in the first to ninth embodiment maybe combined, or the device having the constitution shown in FIG. 1A andthe piezoelectric ceramic material described in the tenth embodiment maybe combined.

ELEVENTH EMBODIMENT

FIG. 3 shows an injection apparatus according to the eleventh embodimentof the present invention, where a container 31 has an injection hole 33formed at one end thereof, and a needle valve 35 that can open and closethe injection hole 33 is housed in the container 31.

The injection hole 33 is provided with a fuel passage 37 incommunication therewith. The fuel passage 37 is connected to a fuelsource that is provided outside of the apparatus, so as to receivesupply of fuel at a high pressure that remains always constant. When theneedle valve 35 opens the injection hole 33, the fuel that fills thefuel passage 37 is injected at a predetermined level of high pressureinto a fuel chamber of an internal combustion engine that is not shownin the drawings.

The needle valve 35 has an enlarged top portion of a larger diameter soas to serve as a piston 41 that makes sliding motion in a cylinder 39that is formed in the container 31. The piezoelectric actuator 43 ishoused in the container 31.

With the injection apparatus as described above, when the piezoelectricactuator 43 is caused to expand by a voltage applied thereto, the piston41 is pressed so that the needle valve 35 plugs the injection hole 33and shuts off the fuel supply. When the voltage is removed, thepiezoelectric actuator 43 contracts and a Belleville spring 45 pressesback the piston 41 so that the injection hole 33 communicates with thefuel passage 37 thereby allowing the fuel to be injected.

The present invention relates to the multi-layer piezoelectric deviceand the injection apparatus, but is not limited to the embodimentsdescribed above. For example, the present invention can be applied to afuel injection apparatus of automobile engine, liquid ejecting apparatusof ink jet printer or the like or a drive unit used in precisionpositioning device or vibration preventing device for an opticalapparatus, or to sensor devices such as a sensor element mounted incombustion pressure sensor, knocking sensor, acceleration sensor, loadsensor, ultrasound sensor, pressure sensor, yaw rate sensor or the like,or used as a circuit component mounted in piezoelectric gyro,piezoelectric switch, piezoelectric transducer, piezoelectric breaker orthe like, and is also applicable to other purposes, as long as thepiezoelectric characteristic is utilized.

EXAMPLES Example 1

A multi-layer piezoelectric actuator comprising the multi-layerpiezoelectric device of the present invention was fabricated asdescribed below.

First, a calcined powder of a piezoelectric ceramic material constitutedfrom lead titanate zirconate (PbZrO₃—PbTiO₃) as the main component, abinder and a plasticizer were mixed to form a slurry which was formedinto ceramic green sheets that would become the piezoelectric layer 11having thickness of 150 μm by the doctor blade process.

An electrically conductive paste, prepared by adding a binder to thesilver-palladium alloy made with an arbitrary composition, was appliedto one side of the ceramic green sheet by screen printing method to athickness of 3 μm. Then 300 pieces of the ceramic green sheets werestacked and fired at a temperature of 1000° C. K₂CO₃ or Na₂CO₃ powderwas added to the stock materials of the piezoelectric layers 11, theinternal electrodes 12 and the external electrodes 13.

Alkali metal contained in the multi-layer piezoelectric device thus madefrom the sintering material, in the piezoelectric layers, in theinternal electrodes and in the external electrodes was detected by ICPanalysis.

Then a groove measuring 50 μm in depth and 50 μm in width was formed atthe end of the internal electrode located on the side face of the stackin every other layer, by means of a dicing apparatus.

Then 90% by volume of silver powder of flake-like particles having meanparticle size of 2 μm and 10% by volume of amorphous glass powder havingsoftening point of 640° C. containing silicon as the main componenthaving mean particle size of 2 μm were mixed, and 8 weight parts of thismixture was added to 100 weight parts in total of binder, the silverpowder and the glass powder, so as to prepare the electricallyconductive silver-glass paste by fully mixing the powders. Theelectrically conductive silver-glass paste thus prepared was screenprinted onto a release film. After drying, the paste film was peeled offthe release film to obtain a sheet of electrically conductivesilver-glass paste. Density of the green sheet as measured by Archimedesmethod was 6.5 g/cm³.

The sheet of the silver-glass paste was transferred onto the externalelectrode surface of the stack and was baked at 650° C. for 30 minutes,thereby forming the external electrode from the porous electricallyconductive material having three-dimensional mesh structure. Measurementof void ratio of the external electrode by means of image analysisapparatus on a photograph of a cut surface of the external electrodeshowed a void ratio of 40%.

Then lead wires were connected to the external electrodes, and DCelectric field of 3 kV/mm was applied between the positive and negativeexternal electrodes via the lead wires so as to apply polarizationtreatment for 15 minutes, thereby to complete the multi-layerpiezoelectric actuator using the multi-layer piezoelectric device asshown in FIG. 1.

When a DC voltage of 170 V was applied to the multi-layer piezoelectricdevice, it underwent a displacement of 45 μm in the direction ofstacking. Operation test was conducted on this multi-layer piezoelectricdevice by applying an AC voltage varying between 0 V and +170 V atfrequency of 150 Hz at room temperature.

Change in the amount of displacement (μm) of the multi-layerpiezoelectric device after undergoing 1×10⁹ cycles of operation wasmeasured, and was compared with the displacement of the multi-layerpiezoelectric device in the initial state before starting the continuousoperation, so as to calculate the variation (%) of the amount ofdisplacement and deterioration of the multi-layer piezoelectric device.The results are shown in Tables 1 through 4. Table 1 shows the amountsof displacement with various kinds of alkali metal contained in themulti-layer piezoelectric device, Table 2 shows the amounts ofdisplacement with various kinds of alkali metal contained in thepiezoelectric layers, Table 3 shows the amounts of displacement withvarious kinds of alkali metal contained in the internal electrode, andTable 4 shows the amounts of displacement with various kinds of alkalimetal contained in the external electrode.

TABLE 1 Table 1-1 Alkali metal content Kind of Amount of initial inpiezoelectric alkali displacement (μm) = No layer (ppm) metal A *1-1  2Na Piezoelectric material could not be sintered. 1-2 5 Na 45.0 1-3 30 Na45.0 1-4 50 Na 45.0 1-5 70 Na 45.0 1-6 100 Na 45.0 1-7 250 Na 45.0 1-8500 Na 45.0 *1-9  750 Na 45.0  1-10 5 K 45.0  1-11 30 K 45.0  1-12 50 K45.0  1-13 70 K 45.0  1-14 100 K 45.0  1-15 250 K 45.0  1-16 500 K 45.0*1-17 750 K 45.0 Table 1-2 Ratio (%) of change in amount of displacementAmount of displacement after continuous operation (μm) after continuousto initial displacement = No operation (1 × 10⁹) = B | (A − B)/A × 100 |*1-1  — — 1-2 45.0 0.0 1-3 44.9 0.2 1-4 44.8 0.4 1-5 44.6 0.9 1-6 44.60.9 1-7 44.5 1.1 1-8 44.4 1.3 *1-9  44.0 2.2  1-10 45.0 0.0  1-11 44.90.2  1-12 44.8 0.4  1-13 44.6 0.9  1-14 44.6 0.9  1-15 44.5 1.1  1-1644.4 1.3 *1-17 44.0 2.2 *Out of the scope of the present invention.

TABLE 2 Table 2-1 Alkali metal content Kind of Amount of initial ininternal alkali displacement (μm) = No electrode (ppm) metal A *1-18  2Na Piezoelectric material could not be sintered. 1-19 5 Na 45.0 1-20 30Na 45.0 1-21 50 Na 45.0 1-22 70 Na 45.0 1-23 100 Na 45.0 1-24 250 Na45.0 1-25 500 Na 45.0 *1-26  750 Na 45.0 1-27 5 K 45.0 1-28 30 K 45.01-29 50 K 45.0 1-30 70 K 45.0 1-31 100 K 45.0 1-32 250 K 45.0 1-33 500 K45.0 **1-34  750 K 45.0 Table 2-2 Amount of Ratio (%) of change inamount displacement (μm) of displacement after after continuouscontinuous operation to operation (1 × 10⁹) = initial displacement = |(A− No B B)/A × 100 | *1-18  — — 1-19 45.0 0.0 1-20 44.9 0.2 1-21 44.80.4 1-22 44.6 0.9 1-23 44.6 0.9 1-24 44.5 1.1 1-25 44.4 1.3 *1-26  44.02.2 1-27 45.0 0.0 1-28 44.9 0.2 1-29 44.8 0.4 1-30 44.6 0.9 1-31 44.60.9 1-32 44.5 1.1 1-33 44.4 1.3 *1-34  44.0 2.2

TABLE 3 Table 3-1 Alkali metal content in Kind of Amount of initialexternal alkali displacement (μm) = No. electrode (ppm) metal A *1-35  2Na Piezoelectric material could not be sintered. 1-36 5 Na 45.0 1-37 30Na 45.0 1-38 50 Na 45.0 1-39 70 Na 45.0 1-40 100 Na 45.0 1-41 250 Na45.0 1-42 500 Na 45.0 *1-43  750 Na 45.0 1-44 5 K 45.0 1-45 30 K 45.01-46 50 K 45.0 1-47 70 K 45.0 1-48 100 K 45.0 1-49 250 K 45.0 1-50 500 K45.0 *1-51  750 K 45.0 Table 3-2 Amount of Ratio (%) of change in amountdisplacement (μm) of displacement after after continuous continuousoperation to operation (1 × 10⁹) = initial displacement = | (A − No BB)/A × 100 | *1-35  — — 1-36 45.0 0.0 1-37 44.9 0.2 1-38 44.8 0.4 1-3944.6 0.9 1-40 44.6 0.9 1-41 44.5 1.1 1-42 44.4 1.3 *1-43  44.0 2.2 1-4445.0 0.0 1-45 44.9 0.2 1-46 44.8 0.4 1-47 44.6 0.9 1-48 44.6 0.9 1-4944.5 1.1 1-50 44.4 1.3 *1-51  44.0 2.2

TABLE 4 Table 4-1 Alkali metal Amount of initial content in Kind ofalkali displacement (μm) = No. device (ppm) metal A *1-52  2 NaPiezoelectric material could not be sintered. 1-53 5 Na 45.0 1-54 30 Na45.0 1-55 50 Na 45.0 1-56 70 Na 45.0 1-57 100 Na 45.0 1-58 200 Na 45.01-59 300 Na 45.0 *1-60  500 Na 45.0 1-61 5 K 45.0 1-62 30 K 45.0 1-63 50K 45.0 1-64 70 K 45.0 1-65 100 K 45.0 1-66 200 K 45.0 1-67 300 K 45.0*1-68  500 K 45.0 Table 4-2 Amount of Ratio (%) of change indisplacement (μm) amount of displacement after continuous aftercontinuous operation operation (1 × 10⁹) = to initial displacement = NoB | (A − B)/A × 100 | *1-52  — — 1-53 45.0 0.0 1-54 44.9 0.2 1-55 44.80.4 1-56 44.6 0.9 1-57 44.6 0.9 1-58 44.5 1.1 1-59 44.4 1.3 *1-60  44.02.2 1-61 45.0 0.0 1-62 44.9 0.2 1-63 44.8 0.4 1-64 44.6 0.9 1-65 44.60.9 1-66 44.5 1.1 1-67 44.4 1.3 *1-68  44.0 2.2 *Out of the scope of thepresent invention.

From the tables, it can be seen that the ratio of change in displacement(%) rapidly increases and deterioration proceeds when alkali metalcontent in the multi-layer piezoelectric device exceeds 300 ppm. Whenthe alkali metal content is less than 5 ppm, piezoelectric layer couldnot be sintered and function of the piezoelectric layer could not beachieved. Accordingly, a piezoelectric actuator having excellentdurability and high reliability that does not cause malfunction of thedevice can be provided as the amount of displacement does notsubstantially change even when the multi-layer piezoelectric device isoperated over a long period of time, by controlling the alkali metalcontent in the multi-layer piezoelectric device in a range from 5 ppm to300 ppm.

This applies similarly to the contents of alkali metal in thepiezoelectric layers, in the internal electrodes and in the externalelectrodes.

Example 2

Experiments were conducted similarly to Example 1, except for addingtitanium chloride to the piezoelectric layers and adding AgCl to thestock materials of the internal electrodes and the external electrodes.Halogen element contents in the piezoelectric layers, the internalelectrodes and the external electrodes of the multi-layer piezoelectricdevice thus obtained were detected by ion chromatography. Results of theexperiments are shown in Tables 5 through 8.

TABLE 5 Table 5-1 Halogen element content in Kind of Amount of initialpiezoelectric halogen displacement (μm) = No. layer (ppm) element A*2-1  2 Cl Piezoelectric material could not be sintered. 2-2 5 Cl 45.02-3 20 Cl 45.0 2-4 50 Cl 45.0 2-5 70 Cl 45.0 2-6 100 Cl 45.0 2-7 500 Cl45.0 2-8 1500 Cl 45.0 *2-9  2000 Cl 45.0  2-10 5 Br 45.0  2-11 20 Br45.0  2-12 50 Br 45.0  2-13 70 Br 45.0  2-14 100 Br 45.0  2-15 500 Br45.0  2-16 1500 Br 45.0 *2-17 2000 Br 45.0 Table 5-2 Ratio (%) of changein Amount of amount of displacement displacement (μm) after continuousoperation operation (1 × 10⁹) = to initial displacement = No B | (A −B)/A × 100 | *2-1  — — 2-2 45.0 0.0 2-3 44.9 0.2 2-4 44.8 0.4 2-5 44.60.9 2-6 44.6 0.9 2-7 44.5 1.1 2-8 44.4 1.3 *2-9  44.0 2.2  2-10 45.0 0.0 2-11 44.9 0.2  2-12 44.8 0.4  2-13 44.6 0.9  2-14 44.6 0.9  2-15 44.51.1  2-16 44.4 1.3 *2-17 44.0 2.2 *Out of the scope of the presentinvention.

TABLE 6 Table 6-1 Halogen element content in Kind of Amount of initialinternal electrode halogen displacement (μm) = No (ppm) element A *2-18 2 Cl Piezoelectric material could not be sintered. 2-19 5 Cl 45.0 2-2020 Cl 45.0 2-21 50 Cl 45.0 2-22 70 Cl 45.0 2-23 100 Cl 45.0 2-24 500 Cl45.0 2-25 1500 Cl 45.0 *2-26  2000 Cl 45.0 2-27 5 Br 45.0 2-28 20 Br45.0 2-29 50 Br 45.0 2-30 70 Br 45.0 2-31 100 Br 45.0 2-32 500 Br 45.02-33 1500 Br 45.0 *2-34  2000 Br 45.0 Table 6-2 Amount of Ratio (%) ofchange in displacement (μm) amount of displacement after continuousafter continuous operation operation (1 × 10⁹) = to initial displacement= No B | (A − B)/A × 100 | *2-18  — — 2-19 45.0 0.0 2-20 44.9 0.2 2-2144.8 0.4 2-22 44.6 0.9 2-23 44.6 0.9 2-24 44.5 1.1 2-25 44.4 1.3 *2-26 44.0 2.2 2-27 45.0 0.0 2-28 44.9 0.2 2-29 44.8 0.4 2-30 44.6 0.9 2-3144.6 0.9 2-32 44.5 1.1 2-33 44.4 1.3 *2-34  44.0 2.2 *Out of the scopeof the present invention.

TABLE 7 Table 7-1 Halogen element Kind of Amount of initial content inexternal halogen displacement (μm) = No electrode (ppm) element A *2-35 2 Cl Piezoelectric material could not be sintered. 2-36 5 Cl 45.0 2-3720 Cl 45.0 2-38 50 Cl 45.0 2-39 70 Cl 45.0 2-40 100 Cl 45.0 2-41 500 Cl45.0 2-42 1500 Cl 45.0 *2-43  2000 Cl 45.0 2-44 5 Br 45.0 2-45 20 Br45.0 2-46 50 Br 45.0 2-47 70 Br 45.0 2-48 100 Br 45.0 2-49 500 Br 45.02-50 1500 Br 45.0 *2-51  2000 Br 45.0 Table 7-2 Amount of Ratio (%) ofchange in displacement (μm) amount of displacement after continuousafter continuous operation operation (1 × 10⁹) = to initial displacement= No B | (A − B)/A × 100 | *2-35  — — 2-36 45.0 0.0 2-37 44.9 0.2 2-3844.8 0.4 2-39 44.6 0.9 2-40 44.6 0.9 2-41 44.5 1.1 2-42 44.4 1.3 *2-43 44.0 2.2 2-44 45.0 0.0 2-45 44.9 0.2 2-46 44.8 0.4 2-47 44.6 0.9 2-4844.6 0.9 2-49 44.5 1.1 2-50 44.4 1.3 *2-51  44.0 2.2

TABLE 8 Table 8-1 Halogen element Kind of Amount of initial content inhalogen displacement (μm) = No device (ppm) element A *2-52  2 ClPiezoelectric layer could not be sintered. 2-53 5 Cl 45.0 2-54 20 Cl45.0 2-55 50 Cl 45.0 2-56 70 Cl 45.0 2-57 100 Cl 45.0 2-58 500 Cl 45.02-59 1000 Cl 45.0 *2-60  1500 Cl 45.0 2-61 5 Br 45.0 2-62 20 Br 45.02-63 50 Br 45.0 2-64 70 Br 45.0 2-65 100 Br 45.0 2-66 500 Br 45.0 2-671000 Br 45.0 *2-68  1500 Br 45.0 Table 8-2 Amount of Ratio (%) of changein displacement (μm) amount of displacement after continuous aftercontinuous operation operation (1 × 10⁹) = to initial displacement = NoB | (A − B)/A × 100 | *2-52  — — 2-53 45.0 0.0 2-54 44.9 0.2 2-55 44.80.4 2-56 44.6 0.9 2-57 44.6 0.9 2-58 44.5 1.1 2-59 44.4 1.3 *2-60  44.02.2 2-61 45.0 0.0 2-62 44.9 0.2 2-63 44.8 0.4 2-64 44.6 0.9 2-65 44.60.9 2-66 44.5 1.1 2-67 44.4 1.3 *2-68  44.0 2.2

From the tables, it can be seen that the ratio of change in displacement(%) rapidly increases and deterioration proceeds when halogen elementcontent in the multi-layer piezoelectric device exceeds 1000 ppm. Whenthe halogen element content is less than 5 ppm, piezoelectric layercould not be sintered and function of the piezoelectric layer could notbe achieved. Accordingly, a piezoelectric actuator having excellentdurability and high reliability that does not cause malfunction of thedevice can be provided as the amount of displacement does notsubstantially change even when the multi-layer piezoelectric device isoperated over a long period of time, by controlling the alkali metalcontent in the multi-layer piezoelectric device in a range from 5 ppm to300 ppm.

This applies similarly to the contents of halogen element in thepiezoelectric layers, in the internal electrodes and in the externalelectrodes.

Example 3

In Example 3, experiments were conducted similarly to Example 1, byusing multi-layer piezoelectric device having different compositions ofthe internal electrode. The results are shown in Table 9.

TABLE 9 Table 9-1 Pd content Pt content Ag content Ratio (%) of changein metal of in metal of in metal of in amount of internal internalinternal displacement after electrode electrode electrode continuousoperation (% by (% by (% by to initial No weight) weight) weight)displacement 3-1 0 0 100 Destroyed due to migration. 3-2 0.001 0 99.9990.7 3-3 0.01 0 99.99 0.7 3-4 0.1 0 99.9 0.4 3-5 0.5 0 99.5 0.2 3-6 1 099 0.2 3-7 2 0 98 0 3-8 4 1 95 0 3-9 5 0 95 0  3-10 8 0 92 0  3-11 9 091 0.2  3-12 9.5 0 90.5 0.2  3-13 10 0 90 0.4  3-14 15 0 85 0.7  3-15 200 80 0.9  3-16 30 0 70 0.9 Table 9-2 Ratio (%) of change in amount Othermetal of of displacement after internal electrode (% continuousoperation to initial No by weight) displacement 3-17 Cu 100% 0.2 3-18 Cu99.99% 0.1 3-19 Ni 100% 0.4

The above table shows that the multi-layer piezoelectric device wasdestroyed due to silver migration disabling continuous operation whenthe internal electrodes 12 of No. 3-1 were formed from 100% silver.Metal compounds in the internal electrodes 12 of Nos. 3-15 and 3-16contained more than 15% by weight of group VIII metal and less than 85%by weight of group Ib metal, thus resulting in increasing deteriorationduring continuous operation and decreasing durability of the multi-layerpiezoelectric actuator.

Metal compounds in the internal electrodes 12 of Nos. 3-2 through 3-14were controlled so that proportion M1 (% by weight) of the group VIIImetal and proportion M2 (% by weight) of the group Ib metal satisfy therelations 0≦M1≦15, 85≦M2≦100 and M1+M2=100. This enabled it to decreasethe specific resistance of the internal electrode and suppress heat frombeing generated from the internal electrode even when operatedcontinuously, thus achieving the multi-layer piezoelectric actuatorhaving the amount of displacement of the device being stabilized.

The present invention is not limited to the Examples described above,and various modifications may be made within the scope of the presentinvention.

Examples 4 to 6

In Examples 4 through 6, multi-layer piezoelectric actuators comprisingthe multi-layer piezoelectric device were made as described below.

First, a calcined powder of a piezoelectric ceramic material constitutedfrom lead titanate zirconate (PbZrO₃—PbTiO₃) as the main component, abinder and a plasticizer were mixed to form a slurry which was formedinto ceramic green sheets that would become the piezoelectric layer 11having thickness of 150 μm by the doctor blade process.

An electrically conductive paste, prepared by adding a binder to thesilver-palladium alloy made with an arbitrary composition, was appliedto one side of the ceramic green sheet by screen printing method to athickness of 3 μm. Then 300 pieces of the ceramic green sheets werestacked and fired at a temperature of 1000° C.

Then a groove measuring 50 μm in depth and 50 μm in width was formed atthe end of the internal electrode located on the side face of the stackin every other layer, by means of a dicing apparatus.

Then 90% by volume of silver powder of flake-like particles having meanparticle size of 2 μm and 10% by volume of amorphous glass powder havingsoftening point of 640° C. containing silicon as the main componenthaving mean particle size of 2 μm were mixed, and 8 weight parts of thismixture was added to 100 weight parts in total of binder, the silverpowder and the glass powder, so as to prepare the electricallyconductive silver-glass paste by fully mixing the powders. Theelectrically conductive silver-glass paste thus prepared was screenprinted onto a release film. After drying, the paste film was peeled offthe release film to obtain a sheet of electrically conductivesilver-glass paste. Density of the green sheet as measured by Archimedesmethod was 6.5 g/cm³.

The sheet of the silver-glass paste was transferred onto the externalelectrode 15 surface of the stack 13 and was baked at 650° C. for 30minutes, thereby forming the external electrode 15 from the porouselectrically conductive material having three-dimensional meshstructure. Measurement of void ratio of the external electrode 15 bymeans of image analysis apparatus on a photograph of a cut surface ofthe external electrode 15 showed a void ratio of 40%.

Then lead wires were connected to the external electrodes 15, and DCelectric field of 3 kV/mm was applied between the positive and negativeexternal electrodes 15 via the lead wires so as to apply polarizationtreatment for 15 minutes, thereby to complete the multi-layerpiezoelectric actuator using the multi-layer piezoelectric device asshown in FIG. 1.

Example 4

The multi-layer piezoelectric actuator of the present invention was madeby the manufacturing method wherein device resistance and induction loss(tan δ) of the piezoelectric layer 11 were controlled. Changes in thedevice dimension and temperature before and after continuous operationof the multi-layer piezoelectric actuator were measured, and therelation thereof with the change in the amount of displacement beforeand after continuous operation of the multi-layer piezoelectric actuatorwas investigated.

As Comparative Example, sample was made of which change in devicedimension before and after continuous operation of the multi-layerpiezoelectric actuator exceeded 1%.

When a DC voltage of 170 V was applied to the multi-layer piezoelectricactuator made as described above, all the multi-layer piezoelectricactuators underwent displacement of 45 μm in the direction of stacking.Operation test was conducted on the multi-layer piezoelectric actuatorsby applying an AC voltage varying between 0 V and +170 V at frequency of150 Hz at room temperature to carry out continuous operation of 1×10⁹cycles. The results are shown in Table 10.

TABLE 10 Table 10-1 Resistance of internal electrode normalized toAmount of initial No. resistance of 100% silver displacement (μm) = A4-1 2 45.0 4-2 3 45.0 4-3 4 45.0 4-4 5 45.0 4-5 8 45.0 4-6 10 45.0 4-7 545.0 *4-8  10 45.0 Table 10-2 Change (%) in device Change (%) in Amountof temperature device dimension displacement (μm) of after continuousafter continuous device after operation (1 × operation (1 × continuousoperation No. 10⁹ cycles) 10⁹ cycles) (1 × 10⁹ cycles) = B 4-1 0.0 0.045.0 4-2 0.5 0.1 44.9 4-3 0.9 0.2 44.8 4-4 1.4 0.3 44.7 4-5 2.3 0.5 44.54-6 3.2 0.7 44.1 4-7 4.5 1.0 43.5 *4-8  9.0 2.0 Destroyed by thermalexcursion.

From Table 10, it can be seen that change in device dimension of sampleNo. 4-8 made as Comparative Example before and after continuousoperation exceeded 1%, and therefore the amount of displacement aftercontinuous operation significantly decreased. Furthermore, since localheat generation occurred in the junction between the internal electrode12 and the external electrode 15, the multi-layer piezoelectric actuatorwas destroyed by thermal excursion.

Samples Nos. 4-1 through 4-7 made in Examples of the present inventionwere multi-layer piezoelectric actuators made to control the change indevice dimension before and after continuous operation within 1%. Thusthese multi-layer piezoelectric actuators exhibited effective amount ofdisplacement that is required thereof, without significant decrease inthe amount of displacement after continuous operation of 1×10⁹ cycles.The multi-layer piezoelectric actuators also showed excellent durabilitywithout undergoing thermal excursion and malfunction.

Example 5

The multi-layer piezoelectric actuator of the present invention was madeby the manufacturing method described above wherein device resistanceand induction loss (tan δ) of the piezoelectric layer 11 werecontrolled. Changes in thickness of the internal electrode 12 and indevice temperature before and after continuous operation of themulti-layer piezoelectric actuator were measured, and the relationthereof with degree of deterioration represented by the change in theamount of displacement before and after continuous operation of themulti-layer piezoelectric actuator was investigated.

The degree of deterioration is defined as the ratio of the amount ofdisplacement after a predetermined number of cycles (the amount ofdisplacement after continuous operation) of the multi-layerpiezoelectric actuator to the amount of displacement before continuousoperation (the amount of displacement in the initial state). This valueenables it to examine the progress of deterioration that is caused bycontinuous operation of the multi-layer piezoelectric actuator.

As Comparative Example, sample was made of which change in thickness ofthe internal electrode 12 before and after continuous operation of themulti-layer piezoelectric actuator exceeded 5%.

When a DC voltage of 170 V was applied to the multi-layer piezoelectricactuators made as described above, all the multi-layer piezoelectricactuators underwent displacement of 45 μm in the direction of stacking.Operation test was conducted on the multi-layer piezoelectric actuatorsby applying an AC voltage varying between 0 V and +170 V at frequency of150 Hz at room temperature to carry out continuous operation of 1×10⁹cycles. The results are shown in Table 11.

TABLE 11 Table 11-1 Change (%) in Resistance of device internalInduction temperature electrode loss tan δ Amount of before and afternormalized to (%) of the initial continuous resistance of piezoelectricdisplacement = operation (1 × No 100% silver material A 10⁹ cycles) 5-12 0.5 45.0 0.0 5-2 3 0.5 45.0 0.5 5-3 4 0.5 45.0 0.9 5-4 5 0.5 45.0 1.45-5 8 0.5 45.0 2.3 5-6 10 0.5 45.0 3.2 5-7 5 1.5 45.0 4.5 *5-8  8 1.545.0 5.4 *5-9  10 2.5 45.0 9.0 Table 11-2 Change (%) in Ratio (%) ofchange in thickness of Amount of amount of displacement internaldisplacement after continuous electrode before (μm) after operation tothe and after continuous initial state = | (A − continuous operation (1× B)/A × 100 | = operation (1 × 10⁹ cycles) = Degree of No. 10⁹ cycles)B deterioration (%) 5-1 0.0 45.0 0 5-2 0.5 44.9 0.2 5-3 1.0 44.8 0.4 5-41.5 44.7 0.7 5-5 2.5 44.5 1.1 5-6 3.5 44.1 2.0 5-7 5.0 43.5 3.3 *5-8 6.0 41.4 8.0 *5-9  10.0 Destroyed by — thermal excursion.

From Table 11, it can be seen that change in thickness of the internalelectrode 12 of samples Nos. 5-8 and 5-9 made as Comparative Examplebefore and after continuous operation exceeded 5%, and therefore theamount of displacement after continuous operation significantlydecreased, while the degree of deterioration increased. In sample No.5-9, oxidation swelling was accelerated by significant heat generationfrom the internal electrode 12, resulting in thermal excursion thatdestroyed the multi-layer piezoelectric actuator.

Samples Nos. 5-1 through 5-7 made in Examples of the present inventionwere multi-layer piezoelectric actuators that were constituted tocontrol the change in thickness of the internal electrode 12 before andafter continuous operation within 5%. Thus these multi-layerpiezoelectric actuators exhibited effective amount of displacement thatis required thereof, without significant decrease in the amount ofdisplacement after continuous operation of 1×10⁹ cycles. The multi-layerpiezoelectric actuators also showed excellent durability withoutundergoing thermal excursion and malfunction.

Example 6

Maximum change in the amount of displacement of the multi-layerpiezoelectric actuators having the internal electrodes 12 formed indifferent compositions, that were made by the manufacturing methoddescribed above, was measured during continuous operation, andrelationship between the composition of the internal electrode 12 andthe degree of deterioration of the multi-layer piezoelectric actuatorsafter continuous operation was examined.

When a DC voltage of 170 V was applied to the multi-layer piezoelectricactuators made as described above, all the multi-layer piezoelectricactuators underwent displacement of 45 μm in the direction of stacking.Operation test was conducted on the multi-layer piezoelectric actuatorsby applying an AC voltage varying between 0 V and +170 V at frequency of150 Hz at room temperature to carry out continuous operation of 1×10⁹cycles. The results are shown in Table 12.

TABLE 12 Table 12-1 Pd content in Pt content in Ag content in metal ofmetal of metal of internal internal internal electrode electrodeelectrode No (% by weight) (% by weight) (% by weight) 6-1  0 0 100 6-2 0.001 0 99.999 6-3  0.01 0 99.99 6-4  0.1 0 99.9 6-5  0.5 0 99.5 6-6  10 99 6-7  2 0 98 6-8  4 1 95 6-9  5 0 95 6-10 8 0 92 6-11 9 0 91 6-129.5 0 90.5 6-13 10 0 90 6-14 15 0 85 6-15 0 0 0 6-16 0 0 0 6-17 0 0 06-18 20 0 80 6-19 30 0 70 Table 12-2 Cu content Ni content Ratio (%) ofchange in in metal of in metal of amount of displacement internalinternal after continuous operation electrode electrode to the initialstate = (% by (% by Degree of deterioration No weight) weight) (%) 6-1 0 0 Destroyed due to migration. 6-2  0 0 0.70 6-3  0 0 0.70 6-4  0 00.40 6-5  0 0 0.20 6-6  0 0 0.20 6-7  0 0 0.00 6-8  0 0 0.00 6-9  0 00.00 6-10 0 0 0.00 6-11 0 0 0.20 6-12 0 0 0.20 6-13 0 0 0.40 6-14 0 00.70 6-15 99.9 0.1 0.00 6-16 100 0 0.20 6-17 0 100 0.40 6-18 0 0 1.106-19 0 0 1.10

Table 12 shows that the multi-layer piezoelectric actuator of sample No.6-1 was damaged by silver migration that occurred because the internalelectrode 12 was formed from 100% silver, thus making it difficult tocarry out the continuous operation.

Metal compounds in the internal electrodes 12 of samples Nos. 6-18 and6-19 contained more than 15% by weight of group VIII metal and less than85% by weight of group Ib metal, resulting in increasing deteriorationduring continuous operation and decreasing durability of the multi-layerpiezoelectric actuator.

Metal compounds in the internal electrodes 12 of samples Nos. 6-2through 6-15 were controlled so that proportion M1 (% by weight) of thegroup VIII metal and proportion M2 (% by weight) of the group Ib metalsatisfy the relations 0<M1≦15, 85≦M2<100 and M1+M2=100. This enabled itto decrease the specific resistance of the internal electrode 12 andsuppress heat from being generated from the internal electrode 12 evenwhen operated continuously, thus achieving the multi-layer piezoelectricactuator having the amount of displacement of the device beingstabilized after continuous operation.

In samples Nos. 6-15 through 6-17, the internal electrodes 12 wereformed by using Ni as the group VIII metal and Cu as the group Ib metalas the main component, and therefore specific resistance of the internalelectrode 12 could be kept low so as to suppress heat from beinggenerated from the internal electrode 12 even when operatedcontinuously, thus achieving the multi-layer piezoelectric actuatorhaving the amount of displacement of the device being stabilized aftercontinuous operation.

The present invention is not limited to the Examples described above,and various modifications may be made within the scope of the presentinvention.

Example 7

Predetermined quantities of stock materials are weighed so as make asintered material having composition ofPb_(1-x)Ba_(x)(Zn_(1/3)Nb_(2/3))_(a)(Yb_(1/2)Nb_(1/2))_(b)(Co_(1/3)Nb_(2/3))_(c)(Fe_(2/3)W_(1/3))_(d)Nb_(e)[Zr_(0.48)Ti_(0.52)]_(1-a-b-c-d-e)O₃(x, a, b, c, d and e are values smaller than 1), and predeterminedquantity of SiO₂ is weighed. While the stock material contains SiO₂ asthe impurity, materials that include as less impurity as possible areused. After mixing the materials in wet process in a ball mill for 18hours, the mixture is calcined at 900° C. for 2 hours. The calcinedmaterial is then crushed in wet process by means of ball mill or thelike.

The crushed material was mixed with an organic binder and a plasticizerto form a slurry which was formed into green sheets having thickness of150 μm by the slip casting process. An electrically conductive pasteformed from 90% by weight of Ag and 10% by weight of Pd was applied tothe green sheet by screen printing method to a thickness of 4 μm anddried. Then 20 pieces (200 pieces when measuring piezoelectric strainconstant d₃₃) of the green sheets having the electrode films formedthereon were stacked, and 10 pieces of green sheets without electricallyconductive paste formed thereon were stacked on both ends of the stackin the stacking direction.

The stack was pressurized while heating to 100° C. so as to consolidatethe stack, and was cut into 10 mm square shape. The stack thus cut intothe predetermined dimensions iwa heated to 800° C. for 10 hours toremove the binder, then fired at 1000° C. thereby to make thecolumn-like stack indicated as 1 a. Grooves measuring 100 μm in depthand 50 μm in width in the stacking direction were formed on one sideface of the multi-layer piezoelectric device in one layer and on theother face of the multi-layer piezoelectric device in the next layer andso on, in a staggered configuration, and the grooves were filled withsilicone rubber as an insulator.

Then an electrically conductive thermosetting material was formed in aband shape as the external electrode on the other end of the electrodethat is not insulated, and was subjected to heat treatment at 200° C.Lead wires were connected to the positive external electrode and thenegative external electrode. After coating the circumferential surfaceof the multi-layer piezoelectric device with silicon rubber by dipping,a voltage of 1 kV was applied so as to apply polarization treatment tothe multi-layer piezoelectric device as a whole, thereby making themulti-layer piezoelectric device of the present invention as shown inFIG. 4. Amount of displacement of the multi-layer piezoelectric deviceunder voltage from 0 to 200 V and piezoelectric strain constant d₃₃ weremeasured. The amount of displacement was measured by securing thesample, that was lined with an aluminum foil on the top surface thereof,on a vibration-proof bench, applying voltages from 0 to 200 V to thesample, and averaging the values measured at the center and three pointsalong the periphery of the device by a laser displacement meter.Piezoelectric strain constant d₃₃ was calculated from the number ofstacked layers n=200, displacement ΔL and applied voltage V=200 V usingthe equation d₃₃=ΔL/n·V.

Si content was quantitatively determined by ICP mass spectroscopy withcapability of measuring on ppm order. Thickness of the grain boundarylayer was determined from TEM image.

Tendency of grains to fall off was determined by applying ultrasoniccleaning to the device in pure water for 10 minutes and observing thesurface with a metallurgical microscope thereafter.

Chronic change in the insulation resistance was determined by measuringthe time before the leakage current increases reaching insulationbreakdown in HALT test (Highly Accelerated Life Testing). The test wasconducted in ambient temperature of 300° C. with electric field of 2kV/mm.

The results are shown in Table 13.

TABLE 13 Table 13-1 Composition 1 −a − b − Sample 1 − x x a b c d e c −d − e No. (mol) (mol) (mol) (mol) (mol) (mol) (mol) (mol) *7-1 0.95 0.050.08 0.04 0.04 0.03 0.005 0.805 7-2 0.95 0.05 0.08 0.04 0.04 0.03 0.0050.805 7-3 0.95 0.05 0.08 0.04 0.04 0.03 0.005 0.805 7-4 0.95 0.05 0.080.04 0.04 0.03 0.005 0.805 7-5 0.95 0.05 0.08 0.04 0.04 0.03 0.005 0.8057-6 0.95 0.05 0.08 0.04 0.04 0.03 0.005 0.805 7-7 0.95 0.05 0.08 0.040.04 0.03 0.005 0.805 *7-8 0.95 0.05 0.08 0.04 0.04 0.03 0.005 0.805*7-9 0.95 0.05 0.08 0.04 0.04 0.03 0.005 0.805 *7-10 0.955 0.045 0.0850.04 0.05 0.025 0.005 0.795 *7-11 0.955 0.045 0.085 0.04 0.05 0.0250.005 0.795 7-12 0.955 0.045 0.085 0.04 0.05 0.025 0.005 0.795 7-130.955 0.045 0.085 0.04 0.05 0.025 0.005 0.795 7-14 0.955 0.045 0.0850.04 0.05 0.025 0.005 0.795 *7-15 0.955 0.045 0.085 0.04 0.05 0.0250.005 0.795 Table 13-2 Time before Thickness Si insulation Grain ofgrain content breakdown fall-off boundary No. (ppm) d33 (pm/V) (Hr)resistance layer (nm) *7-1 150 802 63 ∘ 1.1 7-2 90 810 110 ∘ 0.8 7-3 70814 114 ∘ 0 7-4 50 806 123 ∘ 0 7-5 30 812 130 ∘ 0 7-6 10 801 126 ∘ 0 7-75 793 129 ∘ 0 *7-8 3 790 124 x 0 *7-9 0 790 118 x 0 *7-10 200 760 32 ∘1.4 *7-11 100 755 56 ∘ 1.1 7-12 90 758 102 ∘ 0.4 7-13 50 753 100 ∘ 07-14 10 740 106 ∘ 0 *7-15 0 734 103 x 0

As can be seen from Table 13, thickness of the grain boundary layerexceeded 1 nm and insulation breakdown occurred earlier in samples Nos.7-1, 7-10 and 7-11 where 100 ppm or more Si content was contained. Insamples Nos. 7-8, 7-9 and 7-15 where Si content was less than 5 ppm,grain fall-off resistance was low and fall-off of grains was observedafter machining or ultrasonic cleaning. It can be seen that thickness ofthe grain boundary layer becomes 1 nm or less and the volumetricspecific resistance did not change over a long period of time andinsulation breakdown is not less likely to occur even when a voltage isapplied, in case Si content was in a range from 5 ppm to 100 ppm, whichwas within the scope of the present invention.

In the manufacturing method described above, predetermined amount ofSiO₂ is added because it makes it easier to control the composition.However, a starting material that includes much Si content as impuritymay be used without intentionally adding SiO₂, if Si content in thepiezoelectric ceramic material after sintering can be controlled in arange from 5 ppm or more and less than 100 ppm.

INDUSTRIAL APPLICABILITY

The multi-layer piezoelectric device of the present invention can beused and has excellent durability in continuous operation over a longperiod of time under a high voltage and a high pressure, as described sofar, and provides very high value of industrial utility for applicationsin harsh operating environment such as fuel injection apparatus forautomobile.

1. A multi-layer piezoelectric device made by stacking piezoelectriclayers and internal electrodes alternately one on another, wherein thepiezoelectric layer contains PbTiO₃—PbZrO₃ as a main component andcontains Si of not less than 5 ppm and less than 100 ppm; wherein thegrain boundary has single molecules of SiO₂ and has no glass phasecontaining SiO₂.
 2. The multi-layer piezoelectric device according to 1;wherein Si is segregated in the crystal grain boundary and thickness ofthe grain boundary is not larger than 1 nm.